OK since you want to play it in Betaplayer then the obvious choice is to convert it to DivX or XviD. The simplest methods are either to use an automated tool like
WinAVI Video Converter which I can't personally vouch for as I haven't used it or the old VirtualDub method as detailed
here. If you decide to use the second method and get stuck give me a yell and I should be able to get you unstuck.