As far as I know, there isn't any MPEG2 decoder available for free, due to licensing issues.
MPC has something like a decoder built-in, like VLC has also. So both can play DVD video without the need to install external decoders. But if there is a MPEG2 decoder already installed, you can configure MPC to use that.
P.S.: Media Player Classic (MPC) has nothing to do with Windows Media Player (WMP), although it looks like WMP 6.x