Those free ones are crap,I use BPS and Trend Micro along with all the popular free ones. here's some links to some freebies
CLEAN:
hahaha8226; HijackThis (HJT) / Tutorial
hahaha8226; Ad Aware / Tutorial
hahaha8226; SpySweeper
hahaha8226; Spybot S&D / Tutorial
hahaha8226; CWShredder
hahaha8226; The Cleaner
PREVENT:
hahaha8226; SpywareBlaster
hahaha8226; SpywareGuard
ANTIVIRUS & FIREWALL:
hahaha8226; Kaspersky AV
hahaha8226; Sygate Firewall Pro
hahaha8226; Kerio Firewall
hahaha8226; Zone Alarm
ONLINE VIRUS SCANNERS:
hahaha8226; TrendMicro
hahaha8226; BitDefender
hahaha8226; Panda
hahaha8226; RAV