Two important programs that I have on my Utilities CD, and use quite often, are the Norton removal Tool and the McAfee removal tool.

Another tool I like to use to get rid of difficult programs, is the "REVO Uninstaller, FREE".
It not only uninstalls the program, but also gets rid of the folders and registry entries that many programs leave behind when they UN-Install.

Windows Defender will not (usually) run till all traces of the old AV software are removed.
Even some Anti-Spyware software, like Spybot S&D 1.62, will prevent Windows Defender from starting.

Good Luck!