Trojans/worms/virus - use antivirus
spyware/hijackers - use anti-spyware
So basically if you have a problem run them both. Larry posted some links to Housecall, which is pretty good, and Panda in another thread. Use them if you don't have an antivirus. The following AV are free:
AVG antivirus
Avast! antivirus
Antivir
Bitdefender
VCatch
Don't forget to get a firewall while you're at it! It is as important as a good antivirus. Some free good ones:
Kerio Personal Firewall
Outpost Firewall
Sygate Personal Firewall
Sheesh, that wasn't the funniest thing to do.