Link tag fixed.
Try the VLC media player. It's user interface is a bit unorthodox compared to for example Windows Media Player, but as an advantage it plays almost anything you can throw at it without much problems. It doesn't like Real video, but that's a special case - almost nothing except real can playback real anyways...
But VLC is good to try, as it plays QuickTime, WMV, XVid/DivX-AVI, other kinds of AVI, MPEGs, Theora, DVD's, VCD's etc. without any additional codecs or codec packs installed on the PC.
CCCP is the option to try if you dislike VLC or it doesn't solve the problem. It will install FFDShow, which is a DirectShow codec pack that contains pretty much everything that is needed in video/audio playback in most file types. That works fine most of the time, too.