This sometimes happened to me when I clicked on VS2008 express projects after installing VS2008 shell, since it registered those project files as VS2008 shell projects. The simple workaround was to just start up the correct VS2008 and open it from inside the program, but it doesn't appear that your issue is the same.
The 2010 CTP shouldn't interfere with a 2008 install as far as I know. There was an issue where having a VS2010 install side-by-side with a CTP install would cause issues, but I think they resolved that a while back (I have them both installed side-by-side with no issues).