I had the same problem and finally got it fixed.

I'm using Vista.

Goto C:\ProgramData\Microsoft\Windows\DRM and delete all of the contents.

Goto your drmdbg folder and delete drmdbg.ini.

Play the movie your having problems with. Media Player should install a new license file. Close Media Player.

Fire up drmdbg and hit the Multiscan folder. (Make sure your video is in the same folder first)

Double click the video in the list if it doesn't start automatically and BANG....you're fixed.

This worked for me. SWEET!