Besides my usual AntiVirus program and my Firewall, I use this against the spyware, adware, hijackers and all that nasty stuff that is snuck on your 'puter while surfing
Spybot Search & Destroy . It detects everything, destroys it, and since a coupla weeks, immunizes your pc, i.e. mends little securityholes. Spybot is for free, and available in many different languages.