I use AVG Anti-Virus, Zone Alarm Fire Wall, Adaware & Sypbot Search & Destroy. They are all free and I am happy with them.
I also use Spyware Blaster and Spyware Guard which are designed to stop spyware from installing in the first place.

Spyware Guard
http://www.javacoolsoftware.com/spywareguard.html

Spyware Blaster
http://www.javacoolsoftware.com/spywareblaster.html

All are freeware programs