Lavasofts Ad-aware Personal is free and a good program to have.
IMO-alot better then norton and the other more well known virus scans.
Mostly everyone at my company uses Ad-aware and here at home I find it picks up on alot of spyware that norton doesn't...go figure.
or....you can just take Dante's advice and just don't run mystery programs.
About twice a month my father will call me about some kind of computer related problem he's having......usually its just spyware picked up by looking at porn, odd emails being opened(people just delete them for cryin out loud), those ever lovable pop-up add's(no offense streethop).....and sometimes just downloading multimedia related stuff and opening them to try and play them will instantly install spyware/viruses without any notification to the "casual" computer user.
stay up to date on your scandisk, disk defrag, virus definitions, and every so often check what processes/tasks you have running when things start going crazy on your computer.
ok, im done now...lol