I got this when I used a special codec that kept playing even if there were screw ups in the avi file and that helped w/ audio synching. It only seemed to make the GOOD avi files play upside down. The bad ones that needed the codec played unaffected (except now they didn't stop when a bad spot came up, they just played through until the video could be resumed).
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DiVX? Xvid?
Get a post-June version of Xvid, and upgrade to the newest DiVX.