There are antivirus solutions available for all kinds of computers and operating systems, though for older hardware or more unusual operating systems the Internet might be the only place to purchase them. For the newest products your PC will need somewhere around 40MB of hard disk space, a Pentium II 233+ processor, Windows 98/NT/2000/ME/XP and an Internet connection for getting updates.
For MacOS systems the newest products require OS X, around 15Mb of disk space, a CDROM and an Internet connection. There are still older products that support System 7.5.5 and above, and some of the newest packages ship including a version that supports MacOS 8.1 and 9.x.
Linux products appear on the whole to be much more particular. There are products available which require RH 6.2 or 7.1, 128Mb RAM, 150Mb space in /opt. Others specify exact libraries required ( eg. glibc-2.1.3, gtk+-1.2.8, glib-1.2.8 and XFree86-libs-3.3.5).
Free vs fee AV
Even though the heyday of the free virus scanner has long since past, there are still some commercial vendors offering free cut-down antivirus products over the Internet (See The Free Site for more details).
There is a catch though - some lack automatic updates, some lack e-mail integration and since free antivirus products are downloaded, all lack CDROMs with scanning software on them (See "Is it too late to buy AV software if I've already got a virus?" below).
With antivirus programs features aren't always the most important thing. The most common difference between any free piece of software and commercial software is the availability and organisation of support. In the case of antivirus programs, this also includes virus signature updates and support.
If you can survive without the bells and whistles available in the commercial software, and can cope with finding your support via other users on bulletin boards, then a freeware antivirus package might do the trick. But be aware that updates won't be released anywhere near as often as for commercial products (which can be as often as every day during an outbreak of worms like the W32.Klez variants). In fact, you'll probably be lucky to get updates once a month. And in the Internet age, once a month might mean hundreds if not thousands of new viruses your PC isn't protected from.
No! Most commercial antivirus products are distributed on CDs that include an executable version of the program. This allows you to boot your infected PC then run a copy of the program from the CDROM, which, as read-only media, can't become infected. Even in the worst case scenario, where a boot sector virus has stopped the system from booting, the antivirus program can be installed on another PC and used to make boot disks with a copy of the scanning engine on it. This could then potentially be used to boot up and repair the PC in case of infection. Without the ability to clean a boot sector virus you'll need to get out your installation CDs and reformat your hard disk.
The truly paranoid will always point to test results that show all antivirus programs miss infections during controlled tests and claim this means the defense-in-depth approach of using multiple virus scanners is required. Though two scanners might have a better success rate, the invasiveness of an antivirus product will almost certainly mean the two scanners will have a very noticeable impact on system performance and stability. Find one product you like and keep it up-to-date instead. Very few virus infected computers had an updated and functional antivirus program running on them prior to infection.
Head for your favorite search engine and look for "Online antivirus". The online scanners will be able to tell you if your computer is infected even if they can't help clean it. Try http://housecall.trendmicro.com/ or http://www.pandasoftware.com/activescan/ or http://www.symantec.com/cgi-bin/securitycheck.cgi