An easy solution that can help your MP3 file to play in WMP is to open it with iTunes and in the Advanced menu click Convert Selection to MP3. Then after the file is done converting open it again with WMP. It should now be able to play. This doesn't fix the missing codec problem, but it does enable your MP3 file to play in WMP. I tried your suggestion and can now listen to the MP3 music that the WMP originally balked on. Cool, I'm glad I was of some help. But just a little advice, like I said in my last post, this doesn't fix your missing MPEG-1 (50) codec problem...which really shouldn't be ignored...any problem/error for that matter. As zachd had said before me, the codec should already be on your system and that the error is most likely caused by a 3rd party multimedia program.
"Programming is like sex: one mistake and you're providing support for a lifetime."