This may be irrelevant, but it came to mind.
I installed a bad program that mucked up my file associations (the "MKW Audio Compression tool." It was a good reminder not to install things that I don't know about.) I removed it and cleaned it out of the registry manually to get rid of artifacts, shell extensions, and other garbage it left behind, but it had one side effect on MC.
(And let me emphasize - I don't consider this to be an MC problem. MC is a bright shining jewel.)
When I click on an mp3 file in Windows Explorer, I get an Information window: "Error restoring Media Center license. Failed extracting license from file."
When I right-click and send the mp3 to MC with "Add to playing now" or the like, it goes fine. When I'm in MC and browse within it to My Computer, it goes fine.
I've restored the license, and dropped the file association from MC and reacquired it. Same error. I haven't completely uninstalled and reinstalled MC because it doesn't matter that much to me.
So I mention it in case it gives you any ideas of another place to look in the registry for license problems. For what it's worth!