There are three free anti-virus worth looking at.

Microsoft Security Essentials (www.microsoft.com/security)
AVG Free Edition (http://free.avg.com)
Avast (http://www.avast.com)

I like the Microsoft product the best. It is very quiet, updates itself and has not let anything through yet.

In fact I just recently purchased a new laptop (really kickass specs) and they said I would get 15 months of McAffee. I asked them how much extra to not have it on at all.

(this is where Ross gets on his soapbox)
But the standard caveat applies --- virus protection is two-fold;
1) your behavior --- guess what, your best friend is not trying to send you neekid pictures of whoever is the big star of the day, don't open it.
2) keeping your system up to date -- microsoft spends lots of money on their patch process. Patch Tuesday (2nd tuesday of each month) has a bunch of patches that you should install. Let them install... also if they send an out-of-band patch (on a day other than the patch day) its probably extra important.