High quality movies cause lag :/

Dear boys & girls,

The title says it all...

When I'm watching movies from high quality (f.e. the ET Allstars Trialer and other high quality movies), the movie itself is lagging and sometimes freezing, while the music in the movie just continues...
Got this problem with every movieplayer... VLC, divX, WMP, Winamp, etc...

Anyone got a solution for this? (format doesn't help ;-))

Thnx in advance.