Make sure you don't have "extra" processes taking up your computing resources. (a.k.a. spyware--run SpybotS&D, and Ad-Aware)
Make sure your video acceleration is all the way up. In Windows Media Player, go to TOOLS--Options--Performance, make sure Video Acceleration is at Full.
Try closing unnecessary programs before beginning playback.
This is a longshot, but try making sure your video card drivers and DIRECTX are up to date.
Lastly, uninstall whatever video codecs(xvid, divx, etc) you have and download the DEFILERPAK (
http://hellninjacommando.com/defilerpak/), which should let you play just about anything.
If none of these work, tell me and I'll think of some more.