Switch to VLC Media Player. It plays everything, is free, and is stable.
It didn't use to be that way prior to switching to Win 7 Pro a few years ago. I have uninstalled and re-installed to the latest version more than once, but the issues still persist. Long lag times starting a video or a song, occasionally freezing up and requiring an exit from the task manager.
With that said, I'll add Media Player Classic as another suggested substitute.
Can you access your music and video collections via their folders and subfolders? Because, with Windows Media Player, you have to create playlists within the program. You can't organize your M&V collections by folders and subfolders.