I recommend Microsoft Security essentials, free and great protection. Uses the same scanning engine as their server products (forefront)
I run an IT company and have seen a few viruses get past other free offerings like avast and AVG lately, especially the new "scan HDD" malware and fake AV (av 2010 etc)
I used to recommend ESET Nod32 but I find security essentials to be just as good and free. Microsoft are taking the bad rep of windows being easy to infect and have beefed up security lately with win 7 and they are taking security essentials seriously too.
Unless it's Carmen Electra calling for an emergency titty f**k I'm not here