Spyware is a nasty, malicious presence everywhere that the internet reaches and most importantly on your own PC.
I know little about technical issues and so in these matters I go mostly by recommendation either by brand name (twice the price for an average product) or from someone who really knows. Brett Christensen is one such who writes regularly on computer security matters in his Hoax Slayer newsletter.
So I’m not going to try to pretend I know something that I do not. Just recommend that you go over to http://www.hoax-slayer.com/issue-82.shtml to read about his recommendation for Spyware Doctor.