I've recently cleaned out a mass of spyware from Gator, AdAware and Panicware from a couple of laptops. The stuff was residing in readily readable Folders and when the users were on line these programs were quietly communicating with home base
Other than that, I don't know much about the subject, but when something is Free I usually wonder why.
Cancelling pop-ups is much less work than cleaning out a mess of spyware. Keep an eye on your task-bar to see what is going on behind the page you are looking at and stealing your precious bandwith.