If your system is properly locked down with all unneeded services turned off and software kept updated, none of them should find anything you don't already know about. I run Ad-Aware free and Spybot and neither of them has ever found anything to complain about except the MRU list and a cookie from Excite.com where my main on-line based email is.
I run (and install on all systems I build or work on):
Google Toolbar for pop-up blocking
Edited Host file for ad blocking
Just FYI, anyone running the new Ad-Aware 2007 might want to check Task Manager. The new version leaves its service running continuously even when the program is shut down. It's sucking up 24-35 megs of memory and doing nothing since the free version has no "real time" protection. If you change its service to manual instead of auto it won't run until you activate the program though it will stay resident afterwards unless you reboot or manually stop it.