Anti-Virus Software
avast! antivirus

The “avast! antivirus” has many different versions with support for many computers and operating systems. Some “Home” versions are released with annual registration (and re-registration) that is free of charge for non-commercial home use. It is recommended to check the avast! Latest Program Version Matrix before downloading any of the hyperlinked versions. This is because several products have home pages with major version numbers in their URL, and so it wouldn't be surprising if newer software versions may have newer home pages.

avast! Virus Cleaner
Although this probably has the same compatibility as the “avast! Home Edition”, the only requirements found for this was “the Cleaner requires administrator privileges when running on Windows NT/2000/XP/2003 operating systems.” It did reference trying to scan files with the .CMD extention “as well” on these operating systems, which indicated support for more. When running the program in Win98SE, the About screen was found to refer to Windows Me (suggesting the disabling of automated “System Recovery”). This product may not remove as many different viruses as other products by avast!, and this does not seem to have any support for downloading updated signatures (other than having a user manually check with a web browser to see if a new version has been released), and it does not have real-time scanning listed as a supported feature, although it is completely free (without any registration required).
Versions of avast! for Microsoft Windows
For Windows 95/98/Me/NT4/2000/XP/Vista 32-bit releases, Vista 64-bit, and X64
avast! 4 Home Edition

According to the product's home page, “avast! 4 Home Edition is free of charge for non-commercial home use“. It appears that free annual registration is required (as noted in more detail in the section of the “avast! Linux Home Edition”).

Although the home version download may identify itself as the “Home/Professional Edition”, the download links for the home version and the professional shareware version are in fact different (and point to files with different file sizes).

When the system requirements for avast! were checked online with version 4.8, they were found to match this lenient description that was found in the installation program's output: “avast! 4.8 antivirus protection” “Home/Professional Edition” “for 32-bit Windows operating systems: Windows 95/98/Me/NT4/2000/XP/Vista and 64-bit version of Windows XP/Vista.” The avast! Antivirus and Windows x64 Edition page says “that all avast! antivirus editions, including the free Home Edition, now fully support the new Windows x64 platform (avast! Server Edition supports both x64 and IA64 (Intel Itanium) platforms).” That parenthesied note sounds important for IA64 users if a separate “Server Edition” is needed.

Version 4.8 added support for anti-spyware protection and, for Windows 2000/XP/2003/Vista/2008, anti-rootkit protection. (The other features of the avira! antivirus “Version comparison” page that was noted as being limited to operating systems were “Self-defense (protection of antivirus processes, files and registry keys)”, which had the same operating system requirements as anti-rootkit and was listed on the “What's new in avast! 4.8” page along with anti-spyare and anti-rootkit, and boot-time scanning in 32-bit versions of WIndows NT/2000/XP/2003/Vista/2008.

avast! 4 Professional

To keep this section brief, this paragraph discusses differences from the “avast! 4 Home Edition” product. The description of that product contains details, such as system requirements, which are likely to apply to this version as well. avast! 4 Professional is shareware, according to avast! antivirus - Versions comparison. It also supports additional features, such as a command line scanner and storing scan results in a history. avast! 4 Professional Pricelist

avast! Professional Family Pack
This may be the best option for home use (including commercial home use) if one wishes to purchase at least two copies. The hyperlinked page says “Full non-profit, educational, government and healthcare discounting is available.”
avast! Distributed Network Manager
The console simply requires “Any Windows based OS (Widnows 95 or higher)” while the server, which involves SQL, needs Windows 2000 (Professional or Server), XP, or Windows 2003 Server.
Although the system requirements pages for these versions may not list X64, the avast! Antivirus and Windows x64 Edition page does say “that all avast! antivirus editions, including the free Home Edition, now fully support the new Windows x64 platform”. (This does not include IA64: For that, see the “Server Edition” for Windows.)
For Windows NT 4 Server, Windows 2000 Server, Windows 2003 Server, IA64 platforms
(One would presume this will also support Windows 2008 Server too.) avast! 4 Server Edition supports these operating systems. The avast! Server Edition System Requirements page lists NT4 Server, Windows 2000 Server, and Windows 2003 Server. The avast! Antivirus and Windows x64 Edition page says “avast! Server Edition supports both x64 and IA64 (Intel Itanium) platforms”. (X64 is also supported by the non-server products.)
SBS
avast! 4 SBS is referred to by the System Requirements for the WHS version of avast! WHS version: “avast! WHS Edition cannot be installed under Windows NT/2000/2003/2008 or Windows SBS servers. Such customers should use avast! 4 Server Edition or avast! 4 SBS Edition.” Support for “various server subsystems” are provided via plug-ins which have information on the avast! Server Edition system requirements page.
Windows Home Server
avast! WHS edition
avast! Bootable Antivirus & Recovery Tools CD (BART CD)
This is listed in the Windows section of these product descriptions because of this products descrption on its home page, which says in part, “program now works with all Windows-supported file systems, i.e. NTFS, as well as less common storage devices, such as SCSI/RAID. In general BART will work on all disk devices that Windows XP can work on. Non-standard devices, such as those used in servers, can be handled by standard Windows XP drivers supplied by the hardware vendor.”
More specialized versions
avast! U3 Edition
This is for “U3 USB flash drives” (as described by its main page), which the avast! U3 Edition system requirements page calls a “USB smart drive”. The requirements page says the drive must have 30 MB free for this Edition and this edition has requirements “the same as for the U3 Launchpad itself, i.e. Windows 2000 SP4, Windows XP, Windows 2003 Server or Windows Vista (all editions)”.
avast! for Kerio System requirements for “avast! for Keiro” list requirements for using with Windows NT 4 Service Pack 3 or higher, twice as much RAM needed for Windows 2000/XP/2003, and also the usage of either Kerio Mail Server (version 5.7.3 or higher) or Kerio WinRoute Firewall (version 5.1.8 or higher).
Linux
Free avast! Linux Home Edition
“This software is designed exclusively for home users and non-commercial use. Both of these conditions should be met! Our company offers the Home Edition free of charge, since, in our opinion, it is possible to avoid global virus spreading by efficient prevention; however, many user are not able to or do not want to pay for antivirus software.” “Institutions (even non-commercial ones) are not allowed to use avast! Linux Home Edition.” “Note: the program will not work without a valid license key.” The download page links to a avast! 4 Home Edition registration form which says, “avast! 4 Home Edition works for 60 days after the installation in demo mode. You will obtain your FREE license key by E-mail after the registration. The license key is valid for 1 year. After one year you will need to just re-register.” This Linux version includes a command line scanner (unlike the similarly free Avast! 4 Home Edition for many versions of Microsoft Windows). RPM, DEB, and a *.tar.gz (the latter of which was checked and found to contain at least one ELF binary, and no source code was noticed) are available.
avast! for Linux/Unix Servers
The products page says “avast! for Linux/Unix Servers is an antivirus solution for the more and more popular Linux and BSD environments.” (Specifically, the BSD refers only to FreeBSD.)
FreeBSD
See the avast! for Linux/Unix Servers described in the Linux section for the non-freeware option for FreeBSD.
Other i386 compatible systems
Check out the BART CD in the Windows area.
Mac
avast! for the Mac 60-day trial
PDA devices
System Requirements for avast! PDA Edition refers to a Windows CE version and a PalmOS version.
Any others?
Versions are likely able to be found from avast! Products page, avast! Product Versions page, and/or avast! Beta products, the latter of which has been known to say (on May 4, 2008), “(sorry, there are no beta programs at this time)”.

avast! 4.x VPS page says “Updates are released at least twice a week.” avast! antivirus - Versions comparisons refers to “Incremental updates of virus database (VPS)”. (There is a page with VPS history.) The dates reported on the avast! Latest Program Version Matrix do appear to be in European notation, so 29.10.2007 refers to the 29th of October, and obviously not using the American style notation where such a date entry would look non-sensical, referring to the 10th day of the 29th month. The date on the VPS page matched the one on the “Latest Program Version Matrix” when checked.

The web site largely refers just to the major version number (such as “avast! 4”, including the hyperlinks for the individual downloads (for the home version and the professional version in multiple languages). The best way to ensure the latest version is being obtained may be to check the avast! Latest Program Version Matrix, which has a URL that appears to be independent of any version number, unlike the web pages for some of the individual products (including the freeware and shareware releases for the most mainstream versions of Windows). The latest version number showed up on the left frame. For example, a hyperlink with the quoted description: “What's new in v4.8&dquo; of avast! showed up when version 4.8 was the latest. There was also a hyperlink called Revision History with a URL that seemed specific to the latest major version number. Revision History of avast! 4 gave specific details, including the build number (4.8.1169) and a list of new features. That page could likely be used to effectively check versions as long as the major version number (meaning the part before the first decimal point) hasn't yet changed.

There are numerous languages, including avast! 4 Home Edition: English release.