Windows 7 - Media Player 12 doesn't play WEI movies & causes problems

Hey there.

I've just built a new system and everything "seems" to be working fine but I'm having some problems with WEI and I believe it is related to Media Player 12 playback.

C:\Windows\Performance\WinSAT under this directory there are several movies that are used during WEI tests.
My Media Player (12 I suppose, the one comes with Windows 7) doesn't play them at all. I only see a few frames in the beginning and frozen screen afterwards, although, the movie keeps playing according to progress bar.

I try the same movies with VLC, no problem, fluent as it can be.

This problem causes my WEI to stop and gives an error during Media Codec tests and I cannot turn on AERO, etc.

How can I fix this?
Using the latest Nvidia drivers btw.

Thanks in advance.