First understand that there a WAR out there between Microsoft, Apple, RealNetworks and other media giants in the market. They all want THEIR format to be the DOMINANT format on whatever operating system you're running. That said - barring user patched and unsupported warez CODECs out there - you're NOT going to find a SINGLE CODEC pack that is going to allow you to play ALL formats under Windows Media Player.
There are old "legacy" versions of WMP - which with the proper CODECS installed WILL allow you to play Apple's QuickTime video format as well as RealNetwork's RM formats. This player is very limited in functionality and not supported.
If you're running WMP on Windows XP, I would recommend downloading and instaling the the XP Codec Pack v1.3.4 found at http://www.xpcodcpack.com/
While this will not allow you to play proprietary Apple or Real Networks content - it WILL patch your WMP player with CODECS sufficient to play just about every OTHER type of media that's out there.
Hope this helps.
- John Hook