The problem could be with WMP or it could be with your graphics card. (If you've not got a separate graphics card, that obviously refers your PC's on-board graphics).
The simplest way to find out is to try another media player. The favourite media player of the regular contributors to AB's Technology section is VLC. (It's also been praised by BBC World Service's technology programme). It's free, here:
http://www.videolan.org/vlc/
If your movie files display correctly in VLC, you can either visit the Microsoft website and search for a suitable patch for the problem or (probably simpler) just stick with VLC. (It's much better than WMP, anyway).
If you get the same problem with viewing movies in VLC, you'll know that it's a hardware problem. In which case, if you don't know your way around the inside of a PC, you'll have to find an independent repair shop (unless you really like paying the vastly inflated prices that PC World charge).
Chris