Depending on what you're trying to play, try downloading the Vista Codec Pack.
Otherwise, it could be a corrupted library. Try opening WMP, starting a search for media wherever you keep it, and letting it complete before doing anything else.
realised my last post came off as a tad aggressive. I agree by all means, its just a tad annoying because vista is so much more responsive at 2.7 than 2.4 because i can run CPUFSB:RAMFSB at 3:2 without dropping the frequency of my RAM
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