Great question! There are lots of free antivirus applications, but I've never seen them directly compared. So I did the hard work for you, installing them each in turn on a test machine with about 16GB of data on it and running a full scan of each (including any options to scan inside compressed files). For each application I ran a complete or scan of the hard drive in the application's most thorough, high-priority mode. Otherwise, all default settings were left unchanged.

Below you'll find my notes about the interface, the software in general, and the time it took to run the system scan (plus how many files it actually scoured). While I can't provide much data on how effective these applications are at actually catching real viruses since I don't have a library of viruses to test, I have noted the virus.gr detection rating for each app, which is a good barometer for this. (I've covered these ratings before.) Finally, I've ranked them from my most favorite to least favorite app below. (Note that I'm only covering antivirus software here, not anti-spyware software like Ad-Aware or Spybot.)

AOL Active Virus Shield - TOP PICK! - virus.gr detection rating: 99.62%; total running time: 31 min, 16 sec; total files scanned: 224,565

The bad news: The software requires an email activation code (which means registering online, and which takes a long time to arrive). I also can't blame anyone for not entirely trusting anything with the AOL logo, but Kaspersky (see full review) actually provides the tech behind the antivirus software here, not AOL itself. It also installs an AOL search toolbar in your browser, ugh. The good news: The software is well designed and very easy to use. A progress bar appears during scans, along with estimated time of completion. Everything here looks on the up and up, and the virus.gr detection rating is exemplary. I'm a little perplexed about the missing 70,000 files that Active Virus Shield didn't scan that the other scanners did. (However this isn't necessarily a sign of bad security, as those files may have been identified as not prone to viruses, or Active Virus Shield may count "files" differently.)

Avast! 4.7 Home Edition - virus.gr detection rating: 87.46%; total running time: 49 min, 7 sec; total files scanned: 292,713

I'm not a huge fan of the Avast! Interface, which looks like a MP3 player and features such odd design choices as a "scan thoroughness" slider that looks like a volume control. How do you know how "thorough" this scan really is? Is 4 out of 6 thorough enough? During a scan there's no progress bar, so you never know if you have 3 or 30 minutes remaining. (Plus it's dog slow.) That sub-90% virus.gr rating is not exactly comforting, either. The software comes with a 60 day trial, after which you must register (for free, requires email address). Overall it's just too hard to navigate, which makes it too easy to misconfigure, but it works well enough. (Second opinion: PC World ranked Avast! last among virus scanners it tested in performance at catching real viruses, a fact which the virus.gr rating bears out.)

Avira AntiVir PersonalEdition Classic - virus.gr detection rating: 94.26%; total running time: 14 min, 23 sec; total files scanned: 182,215

AntiVir is relatively popular and you can see why: It's blazing fast, even though it seems to be less than thorough at scanning all the files on the drive. But Avira has at least one terrible feature: Annoying full-screen pop-ups at random intervals advertising Avira's paid software apps. Configuration is about average, and advanced options are available if you dig for them. Overall detection rates look good, making AntiVir a good third choice if the two above don't work for you.

The apps below are not recommended...

BitDefender 8 Free Edition - virus.gr detection rating: 95.57%; total running time: 26 min, 52 sec; total files scanned: 301,157

This is only an on-demand scanner and doesn't offer "real time" protection, which all four of the other scanners in this roundup offer. A decent interface and comprehensive virus.gr rating (not to mention it scanned more files than anything else I tested) don't make up for that unacceptable drawback. I wouldn't recommend it for anyone who needs any level of regular virus protection, even though it's very fast and thorough.

Grisoft AVG Free 7.5 - virus.gr detection rating: 82.82%; total running time: 13 min, 8 sec; total files scanned: 56,391

AVG has a tricky interface, with minimal configurability (for example, you can't specify whether or not AVG should scan within compressed files, and there's no indication of whether or not this happens by default) and no progress bar during scanning. With such a low virus.gr rating I'm not sure I'd trust it as my only line of virus defense. After it finished its scan in only 13 minutes, having scanned a whopping 20% of the files actually on the disk, I was ready to send this one back. (AVG is so popular that I went looking for a second opinion, and sure enough, PC World ranked it second to last among virus scanners it tested.)