WinPatrol

WinPatrol is one of the few programs in this list that isn't free. However, it is inexpensive and provides lifetime registration. There is a trial version that is free, but it doesn't really show the strengths of this program in my humble opinion. It monitors for things that add themselves to your start up programs, IE toolbars, start pages, and other hijacking techniques. In addition it lets you see every application and process that's running (even hidden ones) and will tell you more about it, if you need it, and how to disable it if you want. It lets the system speed up nicely with some tweaking. And when it finds something it barks at you... how much better can it get?!