This has been requested a lot and I don't understand it. Why the hell do people want a build in 3rd class Media Player? Writing a Media Player takes years and a lot of resources as it did for the LimeWire and OpenSource devs to code a P2P application. I mean which tool would you use to cut down a tree? A Swiss Army Knife or a professional tool like a chain saw? No, interoperability is the way to go and not a yet another crappy Media Player with a build in file sharing application.

These two are maybe useful. They support by far more media formats than WMP (WinAmp is as far as I know just an alternative front end for the Windows Media codecs).

http://www.videolan.org
http://www.mplayerhq.hu

0.02€