Mpeg 4 has been made by several companies. DivX, XviD, etc. So DivX codec should still be able to play it. If you want a player that has XviD built into it as well as most others, then get VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)

When you say uncompress it, what do you mean? It's not like a zip file. It's a lossy format like mp3. And what do you want to do with it? Just to play then VLC will do fine. If to burn then Nero & Roxio will also do fine with it. BTW there is a specific XviD codec you can get; XviD (click on link)