There is actually an easier way. I just had the unzipped version of .Net Reflector installed and it had installed itself as a plugin. I exited Visual Studio 2008, deleted the .Net Reflector folder as it is useless, started Visual Studio 2008 and it complained about not being able to load the Add-In and asked if it should be renamed. I let it rename and it was gone. I then went to "C:\\Users\\USERNAME\\documents\\Visual Studio 2008\\Addins" and deleted the renamed file.
FYI, opening the current version and disabling the integration doesn't work because the application won't work.