There are many things that can go wrong when compiling QtCreator (especially on Windows), so I'm not sure exactly what issues you're having. My best guess is it sounds like the QtCore4.dll QtCreator is trying to use at runtime is not compatible with the QtCore4 library being used at link-time, but I'm not sure. Maybe there is an older version of QtCore4.dll on your PATH?

Cheers,
~ andy.f