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AVG Free - AVG has been a personal favorite of mine for a long time. I think that AVG free is probably perfect for the person that wants something that's a jack of all trades. AVG is a very capable tool for scanning for viruses and spyware. It is fairly resource intensive, so you may not want to have it do a full scan daily, but it is great program that you can just set up and forget.
I really like AVG Free for it's extra features though. AVG offers a free LinkScanner program, which scans search results and places a red checkmark beside unsafe sites. It will also stop known attack sites and hacked sites from loading and will require a confirmation before you enter it. AVG also has a fairly nice Resident Shield and E-Mail Scanner that are very nice additions to the package. Overall, it's just nice because it offers a whole lot of anti-spyware and anti-virus protection in a single package.
Avast! -

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Avast! is another good anti-virus and anti-spyware programs. It is primarily an antivirus scanner, but it also has a resident shield that will shut down attack pages and prevent malicious software from loading on your system. Avast has a very simple scanner. You basically just click the button and let the little box scan for a bit. There isn't much room for customization or special features, but it also has a wide range of features that will be great for general protection.
My favorite feature of Avast! is that it offers a boot scan. This will restart your computer and scan it while it boots. This should, ideally, get rid of any spyware or virus that regularly reinstalls itself during the boot up phase. It's really nice to be able to just hit a button and let it do such a full scan.
Avast! also has a very simple button that you can click to set it into gaming mode. In this mode, it won't have any popup alerts or noises. It will just run in the background. This makes it the perfect antivirus software for gamers.
Avira AntiVir -

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Avira AntiVir is another good program. It can also scan for viruses and spyware. It seems like the Antivir Guard is the best of the bunch for proactive virus protection. It was able to stop a variant of the Conflicker worm from getting a foothold awhile ago and has generally been great for spotting malicious processes online.
Note that Avira is probably the most annoying. The nice guard program is the only saving grace. AVG and Avast!, for the most part, just accept that you're using the free version. Avira will issue nearly full screen advertisements for their professional version anytime it updates. This is really annoying and you have to go into the registry to shut it off. It's also terrible for gamers, since the screen can minimize whatever you're playing. I don't like being killed by zombies because my antivirus system can't take a hint.
It also has one of the loudest and most obnoxious alerts for when it finds a problem, although that can be turned off in the menu. This is particularly bad, since Antivir tends to spot a fair number of false positives too (one every few days for me).