Kaspersky Anti Virus 2009.. If you're going free, use Avira..it has the best detection rate of the free AVs.
Complimented by Malwarebytes (which is free).
It doesn't hurt to have Iobit 360 running in the background as well, it won't conflict with your AV. (Also free)
I also use Hitman Pro..It's a free cloud scanner that gets the job done in under 2 minutes. If it hits on something, I'll do a full AV scan. I have Hitman run on start-up.
Might as well top it off by using an outstanding free firewall~~~ZoneAlarm~~~ Far superior, compared to Windows firewall.
Well, this is the package I use/recommend.
Hope it was of use.
pEaCE 