Make sure your screen saver isn't kicking you out of full-screen mode. Also, turning of the Enable full-screen mode switch feature might solve the problem. In Windows Media Player 9 Series or later, do the following:
On the Tools menu, click Options, and then click the Performance tab.
Click Advanced.
Clear the Enable full-screen mode switch check box.

that's not the case cause maybe a week ago it was working and i never changed any option, u think it can be a virus?

I dont suspect a virus here but you should try out the options given in my previous post.

oh im sorry i read that wrong, i thought u meant to make sure Enable Full Screen Mode Switch is checked, not cleared. And sure enough it was checked so thanks it now works.

Marking this thread as solved ...........