I did a Dogpile search for free virus scan and found a lot of information about a lot of different products. The first search result for free virus scan turned up PCTools.com. They offer a product called Spyware Doctor 4.0 that was declared Best of the Year by PC Magazine.

Another result in my Dogpile search for free virus scan took me to a product called Trend Micro House Call. They offer to scan your computer for free while you surf the internet. You must agree to their terms and conditions before you run their program.  Be careful and read all terms and conditions before agreeing to anything.