spybot search and destroy is a fantastic free anti spyware tool.

spywareblaster is good for spyware prevention.

If you're in the market for new antivirus software, Eset Nod32 is great at providing virus/trojan protection AND spyware protection. In fact, since I got Nod32, I don't think I've ever had spybot S&D find anything and I've certainly not had any spyware problems!