When in full screen mode, Media Center uses DirectX "exclusive mode" for rendering to the display - because of this you can't bring a window to the front above Media Center (in fact, if I understand it correctly, technically the window behind isn't there - the "windows" part of Windows is temporarily removed from the display, and replaced with something else).
Generally speaking, it's going to be a bad user experience to do this anyway - the Windows form isn't going to fit in with the Media Center user experience at all. If you really need a WinForm application you could consider launching an entrypoint that runs the WinForm application from within your Media Center application, which will minimise Media Center and run your WinForm app.
Cheers,
Andrew