Just download the Divx player or VLC media player. I prefer VLC personally- it's smaller, faster and can play just about anything...
http://www.divx.com/
http://www.videolan.org/
You can also download them from CNET's download.com website.
http://download.cnet.com/VLC-Media-Player/3000-2139_4-10210434.html?tag=mncol;6
http://download.cnet.com/DivX-Plus-Software/3000-13632_4-10062728.html?tag=main;dropDownForm