One of the ironies of modern computing is the fact that users pay for Microsoft Windows either directly or indirectly, but due to the security holes in Windows most users usually pay again for protection -- most of which should never have been needed in the first place. For example, there are any number of companies lining up to sell you anti-virus, anti-spyware, and firewall software. Even Microsoft itself will sell you a $50-a-year subscription-based product (Microsoft One Care) to fix problems that should never have existed in the first place. This sort of double-dipping certainly doesn't seem fair. Frankly, this is one of several reasons that I mostly use Linux -- while I run it I have no problems with viruses or other malware.
Fortunately, there is free software available that can effectively improve your Windows security just as well as any paid software or service. The trade-off? It takes a little bit more effort to gather it all together and maintain it -- but the knowledge is useful and it does pay off. It's also true that users must be careful, as some anti-malware software is more resource-intensive than others, sucking up CPU cycles like a Dyson at full speed. Combining two such programs can bring even a multi-core system to its metaphorical knees. When this is the case, I will mention it. Also, be aware that there are malicious software programs out there disguised as useful utilities (see here and here for lists and information on some of them).
There are several decent free anti-virus utilities available. Most are effective -- some even more so than the market leaders (though, to be fair and honest, I hear good things about Symantec's soon-to-be-released commercial Norton 2009 suite). There are four that I use on a regular basis, depending upon the version of Windows, the nature of the problem, and the speed of the PC. They are roughly equally effective, though the functional mechanisms do differ. Also be aware that the first three free versions listed are for private home use on one PC only -- institutional and commercial use are not allowed -- though the AVG software (for example) has a fully-functional 30-day trial version available to all. Comodo's package does not have the limitations listed, one of the reasons that I recommend it highly.
One of the better on-line comparisons of the first three free anti-virus utilities discussed here is available here, though it's a bit out-of-date. Rather than go into too much detail in this post, it's a good article to check out as a start. Also realize that most of the packages mentioned here have commercial versions available that are more feature-complete. Buy them or donate if you can afford it; the free versions have to be supported somehow. ClamAV, The last package listed here, has a whole different approach that can be useful, since there's a version that runs under Linux and BSD.
AVG Free Anti-Virus
The newest version of AVG Free Anti-Virus is probably more accurately described as a suite. It includes not only anti-virus software, but also a link-scanner and anti-spyware capabilities. It's what I usually install; the reasons I might install one of the others include being second-machine-on-a-network (the Free-AVG license is limited to one machine per network) and older/slower PCs (it's more of a resource hog than the older versions of AVG were). There is also a version available that runs under Linux; see the ClamAV section to see why this is useful.
Avira AntiVir Personal
This package is nominally the best-performing of the programs listed here (usually) -- but real-world performance has them more evenly matched. One minor gripe is the advertising nag-screen pop-up that is displayed at boot-up, though this is a reasonable attempt to sell the retail product.
avast! antivirus Home Edition
Despite having a somewhat-clunkier user interface than the others (in my opinion), this is still a useful and effective package.
Comodo Internet Security
Note: Comodo AntiVirus is now part of the Comodo Internet Security suite with the firewall integrated. I use this on one of my slower PCs because it rarely sucks up too much in the way of resources -- and I now recommend it over AVG for general use. It seems to be quite effective, with an added bonus: commercial and multi-PC use is OK.
This is the outsider of the bunch, because the philosophy behind it is drastically different from that of the others. It's GPL open-source software that was originally designed to work on Unix and Linux mail servers and gateways to protect downstream Windows-based clients; there's now a version for Windows as well. While they are not kept as up-to-date as some of the other utilities listed here, they are useful tools nonetheless.
Why is a Windows-virus utility that runs on Linux/Unix/BSD useful? For the simple reason that it allows a technician or user to work on Windows from the "outside." Running a virus checker from within Windows is very much like working on an automobile engine while it's running -- there are times it's just easier to work on it when it's turned off. This is made possible by the use of a free "live" Linux CD or DVD such as Knoppix, System Rescue CD, or Trinity Rescue Kit (among others). In an extreme case I may hook the infected Windows-installation hard drive to my own PC running Linux via an external USB adapter.
Since the Clam anti-virus software is an on-demand scanner, it can usually be run under Windows even when other anti-virus software is running in the background.
Part 2 of this series will cover anti-spyware and related software.