Can't play AVI's all of a sudden

Windows XP, Media Player 9, all updated. All of a sudden, I can't play AVI files - when I try to open them in Media Player, I get a message "Media Player cannot play the file - it's either corrupt or doesn't support it." These are files that are definitely not corrupt - have worked many times - home movies from Pinnacle Studio 7 and Movie Maker. They won't play in Pinnacle anymore either. Furthermore, when I try to capture from my video camera in Windows Movie Maker, capture works fine until it tries to open the captured file, then I get a similar error message that the file type is unsupported. I transferred these files to my other win xp computer and they play fine. I've looked through all the codecs and seem to have all of them. I've tried reinstalling Media Player 9 and Pinnacle without success. Please help!
Thanks,
Russ

 

Have you installed any software that may have contained codecs, like Kazaa, Adobe, or anything?

You could try installing all the codecs that are available for WMP9.
http://www.soft32.com/download_2592.html?op=5

If that doesn't work, try removing the codecs and then installing from the download pack.