Also free: a) real time protection: 1) Avast! (AV and anti-Spyware), 2) Spyware Terminator, 3) SpyWareBlaster (actually, passive); b) on demand: 1) Spybot, 2) SUPERAntiSpyware, 3) Windows Defender (wants to run real time but I usually use it on demand).
Best of luck.