Vista - Media center won't start in Vista Ultimate

Having installed Ultimate 64bit, I find that Media center won't start. It
just minimizes to the task-bar and disables the mouse (no mouse cursor seen).
The only way to retain control of the computer is to task-switch to the
desktop, or another running application, then the mouse is back. The
"iconized" media center can only be killed through task-manager.
Inspecting the application log, I find: "A running application prevented
Media Center to change the desktop" - even though no other application is
running.

Any suggestions for solution?

best regards