http://www.spybot.info/en/index.html
Testimonial
CNet:
Spybot delivers useful tools such as an e-mail list of opt-out addresses to stop unwanted solicitations and a file shredder to securely overwrite unwanted files deleted from your PC--all of which Ad-aware Standard Edition lacks. Best of all, Spybot does it all for free. For protection against ad-serving software, Trojan horses, and other means of tracking your surfing habits, get Spybot immediately.
I downloaded free Spybot about 6 months ago and have been trouble free since. Highly recommended!!!!
However, the Spybot writer has stopped doing updates because too many corporations were using his product without sending him any money. I understand he's in the process of turning his free product into a real business. I hope so, I thought it was much better than Ad-Aware.