Norton AntiVirus 2005 - $40/year
Microsoft AntiSpyware beta - Free
AdAware SE Personal - Free
Spybot Search & Destroy - Free
Spyware Blaster - Free
RegSeeker - Free
Norton AV and Microsoft AS are dynamic. They actively monitor your computer and you can scan your system. The freeware versions of the other programs are mostly static. You run these periodically to scan you computer for bad stuff.
What combinations have others found successful?
(...stop me if you've heard it all before - BTE)
Until recently I was using The Proxomitron proxy server which blocked a HUGE amount of pop ups, ads and other crud....but my WinXP didn't like it when it tried to access protected areas of memory. Oh well.