How to fix up the Windows Media Player so that it can play YouTube videos ?

If you are downloading and saving them as FLV file extensions then media player wont play them unless you get a codec pack (K-lite) However I would advise against this as I have had problems in the past with K-lite. The best thing to use is either Media Player Clic Home cinema or VLC Media player. They both support far more media extensions than the standard media player. With Media Player Clic Home cinema you do have to change the file ociations manually through file properties.