F-secure has a free online scanner:
http://support.f-secure.com/enu/home/ols.shtml
It should give a good indication of the state of your system but it's not great at removing a lot of stuff (makes sense, they want you to buy something). If you get a hit on just a couple virii you might want to try googling the names (W32.blahblah) and see if there is a freeware removal tool for it/them, I think it's Mcafee that puts out "Stinger" which will remove a bunch of fairly common ones. If you're heavily infected, which A LOT of machines are these days, you may have to bone up for a current AV product. I've had good luck with Trend-Micro and have seen Norton completely miss common virii/malware. Good luck..