I've started using the latest codec, which is becoming popular on download sites.
It gives better quality in a 40% smaller file size.
Recent VLC, DIVX player, or codec pack should play the files without issue.
Here is a generic codec that will work with Windows Media player:
http://www.strongene.com/en/downloads/downloadCenter.jsp
Ignore the "license price" comments, the decoder is a free download.
There will be 4 DLLs in the folder you downloaded.
Put the one with _x64 in it's name in your Windows\SysWOW64 folder, and the other 3 in your Windows\System32 folder.
Now run the reg.bat and reg64.bat files.
Close and reopen your media player.
If it still doesn't work, download the Lav Filters from here:
http://www.free-codecs.com/download/lav_filters.htm
If you have the Haali splitter installed, heed the notice to uninstall or disable it before installing LAV.