- ClamWin antivirus is an open source antivirus solution
- Spywareterminator has an option to include ClamWin to become an anti spyware/virus combo tool.
- AVG Free is pretty good
- Lavasoft Ad-ware is good
- Spybot is also good
I usually have both Spywareterminator/ClamWin and AVG free both running on my PCs.