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Drag and drop a problem video into GSpot, identify the codec, remove that codec, reboot, re-install. Clicking on System/List Codecs in GSpot will show all the codecs it can find on your system.
You'll need to close any media players/decoders before you can remove some of the codecs, you may even need to reboot in safe mode and delete them then. Use Sherlock for more information about the codecs on your system.