Highly recommend you download the ewido security suite. It scans your PC for "Trojans, Worms, Dialers, Hijackers, Spyware and Keyloggers" that might be missed by your antivirus software. I gave it a try and actually found a bloody trojan horse in my windows folder - it somehow managed to go through my firewall and was undetected by my norton antivirus.
It provides continuous protection for 14 days, after which you have to scan your PC manually which you can do like once a week. Click the title to download it.