Don't be cheap, buy the AV pgm. that suits you. Its too darn important to let it go because you plead no money. Regardless, it pays for itself if you run into problems. Just about all the major vendors have demoware or trailware releases which should cover you for 15-30 days, tops, free. The others that are also shareware, trailware, or freeware to look are Avast, AVG, Nod32, and Fsecure. Google for more or details on these or check out CNet's "Security Forum" here for other leads. Let me repeat, you decide to get only a free version good as they are, they usually don't support you should you run into problems in thier use, but usually that's not a problem, they simply work. You can also use "online scans" should get into problems before installing AV pgm., major vendors support its use.
good luck
-----Willy