One quick way to fix complicated WMP problems is just to reinstall WMP over itself. The player can be downloaded from http://www.microsoft.com/windows/windowsmedia/9Series/GettingStarted//home.asp -- reinstalling it will force all file extensions and associated components to be re-registered. If you want to attempt to troubleshoot the problem manually, there is a very good on-line resource for troubleshooting WMP problems: http://www.nwlink.com/~zachd/pss/pss.html, which contains a great many tips not explicitly documented by Microsoft. I have managed to solve several very thorny problems using this guide.
This was first published in August 2004