From the ones that I have tested, it is really a toss up between AVG
http://free.grisoft.com/doc/1

or

Avast
http://www.avast.com/eng/avast_4_home.html

Some people find Avg lighter on resources than Avast. However, others like Avast's interface and detection better. Some say that Avast is quicker to update than AVG and sometimes due to server issues AVG does not update as quickly.


My suggestion is try one for a while than unistall it and try the other and see which one you like the best. Just don't install both at the same time or your computer might crash.

Also check this post:
http://wiki.castlecops.com/Roll_your_own_Free_Security_Suite

It tells you how to put your own security suite together. This is really important if you have a new computer. There is more to computer security nowadays than just installing a antivirus program. New and emerging spyware is part of the latest threat and just as dangerous if not more dangerous than any viruses out there, so it is equally important to make sure that you download and install anti-spyware detection and prevention programs. The post above provides excellent recommendations.


Hope that helps.