If You want to Play FLV Video files (downloaded from YouTube) with any Video Player,  try FFDshow Video Decoder :

• ffdshow_rev1928_20080410_xxl.exe (Nightly builds by xxl) -> [Download]
• ffdshow_rev1945_20080417_clsid.exe (Nightly builds by clsid) -> [Download]

(Right click on a FLV video file, and chose some Video Player as the default player).