I understand that you already resolved this issue by reformatting but I wanted to give you an idea for a fix if this happens in the future.
It seems as though there is a background program intercepting .exe files when launched and instead launches the media center. My best guess would be to shut down any unnecessary applications using task manager in attempt of closing the program that is causing the interceptions.
Another idea (although not fully recommended) would be to locate the media center folder and delete it. This can create issues with the registry but media center cant launch if its not there The malware intercepting the launchers will probably still be existent however.
I just had the exact same thing happen on my laptop. I recently upgraded to windows 7 and all was working ok and now almost everything I try to open just opens Media Center. Did anyone find out how to fix this without re-loading 7? Any idea about what causes this?