Generally Windows Media Player, (WMP), does not allow Fast Forward of movies. I don't know about Real Player, but even if it does, web based video content can be programmed such that Fast Forward controls are disabled, so it really depends on what and where you are watching videos.

I only know of a couple of media players that allow you to speed up or slow down video playback. The first is the VLC Player from

The second, (if I remember rightly), is the DivX Player from

But as far as I know, the DivX player only plays AVI movies, and it's HD version.