.avi files not recognized as video files

Hi everyone

I'm having a problem with two computers not playing .avi files.

First, let me say that the files are definately good. I can play all of them on my system running Vista Ultimate, and on a friend's system running XP (Im not sure which version of XP that one has). They will not play on two other systems, running Vista Home Premium and XP pro. On both of those, windows media player is updated to the newest version (v. 11, i think), and is set to be the default player for .avi files, and it will play them as audio files, but the video itself is not being recognized.

I know that often, that is a codec problem, but the codecs are being acquired, supposedly - according to the player. When I check the files' properties (from "my computer" ) it is listed as a video file, but when I check the file properties from within the media player, it lists the file as an audio file only. . .

I'm stumped. I dont know what's going on here. At first, I assumed it was just that Vista Ultimate supported the file type, but no other operating system did, but then it worked on my friend's XP system, so now I have no idea.

The two systems that wont play the files, both have dedicated video cards (one ATI 512 mb, and the other Nvidia 512 mb), and the CPUs are both Intel core 2 duos (one 3.0 GHz, the other 2.4 GHz)

Any ideas????

thanks, in advance, for any help!