Stange things happens when i'm trying to play *.avi files

When i tries to open an .avi file i recorded with Fraps screencam, or an avi
file that's an intro to one of my computergames, in Windows Media Player, I
can only hear the sound, and on the screen, where the movie was supposed to
be, there is a big blue note, like if it was a sound-file. But it says that
it's a movie file. And when I tries to open a .avi file that came with the
Vista (C:\windows\system32\migwiz\progress.avi), it plays without problems,
and when i plays the files that doesn't work on my XP, theres no problems. I
don't get any error messages or something - It just doesn't work!!
Please help me