I'll go along with using Firefox with an add-on, but my preference would be for this one:
https://addons.mozill...be-video-downl-10137/
It allows you to save Youtube videos with just a couple of clicks, in your chosen format (such as MP4) so that you don't need to convert from FLV format.
However I'll go along with using VLC (or GOM), in preference to Windows Media Player, anyway; they're far better at playing videos.
Chris
PS: Before someone else points it out, I'll add that saving Youtube videos isn't exactly legal!