I run one but never finds anything. I find spyware with Ad-Aware and Spy Bot. I tried to clean my niece's PC this past weekend. It has over 1,000 pieces of Ad-ware/Spyware and 68 Trojans/Virii. I eliminated about 99% of them with about 10 programs and some command line editing and file removal. If I had the original CD's (restore), it would have been easier to wipe the thing clean and start fresh. It still has several things (causing popups even with XP SP2). Quite annoying.
Anyway, I've seen too many like that to take chances and free McAfee (lifetime! 😉 - not hacked either) works for me! 🙂