Trend Micro HijackThis
Full user review
"Powerful, small and FAST!"
HijackThis is not just a malware detector/destroyer - it has an array of powerful tools. If you have a small hard drive then this is the anti-malware software for you... but een if you have a big hard drive it is still well worth downloading
hijackthis is a powerful tool - and if you click the wrong button you may delete vital Windows files. This could stop your OS working - although when you are making these changes hijack will give you a warning, and there is the option of a restore
Excellent program. Extremely powerful, but also very tiny in size (under 1mb!)
I recommend this to anyone - especially because it is totally FREE