If they're encoded in the MS MPEG4 codec, and that got corrupted, I'm not sure if you can fix it easily short of a full reinstall of Windows - I had that one messed up on my PC once and not even Nimo could repair it. Fortunately, I got a FourCC editor, which basically changes the signatures of an AVI to make it play with a different codec, so I forced them to playback under DivX 3 instead, and they worked, though that doesn't really fix the messed up codec.