WinXp and Media player

Here's the issue:

Acer laptop 1.4 GHz P4..etc.. Hooked up to a KDS 19" monitor.

Recently i've used the windows updater, now when i try to watch a movie it only displays it on the laptop screen, not the monitor. (it worked perfectly fine before)

Example: I can see the windows media player, the desktop, mouse etc but the screen portion is just a black square on the monitor while it plays fine on the laptop.

So far the video formats i've tested are mpeg and avi. I've also tried apple's quicktime player with mixed results. (It plays fine on normal size, if i try double size or fullscreen it goes black again)

I'm under the impression that this is some "digital media protection" software included in my update. Any ideas on how to revert to the previous version or just get rid of it all together?

Thanks.