Windows 7: Background process prevent to have full screen programs running

Hi...I have the following problem. I used to see a movie in full screen using the Wwindows media player and sudenly the screen went out from Full screen like if there was some other program or process trying to do something. This happens every time so I can't see a full screen movie without having to set up the that mode in a permanent basis.

Today I was playing a game also in full screen and suddenly the game screen dissapeared and I got the windows desktop screen and the game was still in the lower bar, meaning that some program or process tried to do something.

It is driving me crazy. Anyone can help me to figure out what might it happens?

Thanksm

Zek