Here are some of my personal thoughts on the best antivirus products on the market, both free and commercial.

Free

Alwil Avast! 4.6 Home Edition - Venerable Avast! is still free for personal use, and it's a relatively unobtrusive and capable virus stomper that will work well for just about anyone. A new feature adds a web proxy to the mix (which filters all traffic running through it for security risks), and it works will all major web browsers. It also recognizing non-virus threats like spyware and adware. The Pro edition ($40) adds a few power-user features that the average user probably won't miss. Give the free version a try!

Aviva AntiVir PersonalEdition Classic 7 - This has long been a capable and effective antivirus product, especially noteworthy for its fast operation. These guys are serious about free protection: They don't even offer a paid version.

Paid

Trend Micro Internet Security Pro - Trend Micro has long been an also-ran in the antivirus world, but after spending some time with a recent version of the software I've been impressed with how well it works, its paucity of crashes, and how generally effective it is. Trend offers three vaguely similar versions of its security application; this one ($70) is the most inclusive, and cheaper than the rest. You can also get plain antivirus for just $40.

Kaspersky Internet Security 7.0 - If you want absolute and utter control over every 1 and 0 that enters and exits your computer, Kaspersky is the app for you. I honestly don't think you can get any more secure than with Kaspersky locking down your system, but it was overkill even for me, and I had trouble convincing it not to remove some applications I knew were OK and actually wanted on my PC. Kaspersky is a hungry shredder with no reverse button (and it's on the slow side, too). $80. (Anti-virus only is $60.)

Symantec Norton 360 - I've been a Norton user in various forms for close to a decade; the latest version doesn't do much different than the last few years' worth, but it's still rock-solid security that I don't hesitate to rely on. My only complaint is that Norton can slow down your computer quite a bit (like Kaspersky) and almost invariably hangs when I'm trying to shut down my computer. If you've ever wondered where all those error messages about programs like "ccApp" not responding come from, well, you've found it. It's frustrating that these problems persist year after year. $80 for Norton 360, or AntiVirus only for $60.