i've the same problem! Every time i open WMP it starts scanning my drive using half of cpu and in the bottom there's "updating media library", or "retrieving media informations for ...." it works for a couple of hours, then it says "update complete", or something similar. If i close WMP and reopen it, it starts scanning again... but the main problem is that it uses a lot of cpu!
Windows 7 runs fine, but media player 12 sucks!!