ClamWin is a free and open source anti-virus solution, that I obviously put my full weight behind. It is pretty good, I have used it before. Oh, if you ever hear the term "Back Door" they mean either:
A) A bug in the code in Windows (which it has a lot of)
B) A open port (internet/virtual port, not the actual ports on the physical computer) or otherwise a security vulnerability which people can exploit. Having a open port is like leaving every door in the house open wide in a large city with a high crime rate, you really don't want that.