wmp10 doesn't play mpeg, it only plays wmv (wonderful microsoft "openness"). I ran into the same problem when I first got my PPC. There are a number of free players that, quite frankly, blow the doors off of wmp10. Take a look around and you will see a number of threads on this site about it.
Before everyone jumps in and says it, let me be the first: go get tcpmp
http://tcpmp.corecodec.org/download
This is the best of the PPC players I have tried and everyone else raves about it as well. You can research all of them (did so myself), but this is probably where you will end up anyways.
Also, its free!