K... If you're on XP use Avira, it's the best free one on the market and has been for a number of years now, google the tests.
If you're on windows 7 just use MSE(Microsoft Security Essentials) which comes with 7. It's genuinely the best out there.
Don't bother with anything else at all. Stay well away from mcafee and norton too. They're pretty fucking close to being defined as malware themselves, pieces of shit.
AV Comparatives do the most comprehensive testing of anti virus software and have the least bias/most scientific approach.
http://www.av-comparatives.org/image...ummary2010.pdf
Page 3 for summarised comparison of the tests. You'll see that Avira is the only free one to receive ADV+ in all categories. F-secure is unfortunately not free and MSE has yet to be retested by them since they made the changes after this comparison study, everywhere else places them as number 1 though.
Sorry guys, AVG has been outdated for years now.