Although this is an old thread, the following information may be a
solution for someone who prefers to use Windows Media Player (WMP).

Use a search engine to download a small program called YouTube
Downloader. With this free download, you will be able to download and
convert any video from YouTube (according to the information described
in the player). When you use this program to convert any previously
downloaded file into WMV file format, you will be able to play the file
on Windows Media Player. Although I am not an expert in these matters, I
have found this a very simple solution for playing .flv files on WMP 11
- presumably this method will work on earlier versions.