If it were me, I'd use avast home edition - http://www.avast.com. its free for home users but the developers ask for a contibution if your using it within a comercial environment, the knock on is that the the definitions are bang upto date because the developers are getting paid.
every time I've had a virus come into my machine (broadband connected, lots of internet/email contact) Avast has spotted it. it's an exellent free product.
IMO It may also be worth looking at a couple of other security features while your at it as well.
try winpatrol - http://www.winpatrol.com - it monitors all running apps and notifies you when any program/file usage changes within your system.
also Ad-Aware SE - http://www.lavasoftusa.com/software/adaware/ - upon a promt form you it scans (deep system or selected directories) for known spyware and tracking software. It updates automaticaly and has always sorted my machine out.
There are sooooooo many products to choose from out there but so far, the combination i'm running (and have been for 1/2 years) has been as water tight as I think your gonna get.
keep it safe!