I suggest uninstalling your faulty installation of Media Player if you can do that. Then, download [urlhttp://www.ccleaner.com]CCleaner[/url] and run it. Download that Basic version unless you want to install Yahoo toolbar which is bundled with the standard version. Take some time to get familiar CCleaner's features before deleting stuff with it. Note that under Applications (under Cleaner Settings), you'll see Windows Media Player specifically mentioned under Multimedia. Be sure it's included, i.e. the box is checked, in your scan. Also, use the Issues button in CCleaner and Scan for Issues.
Disable your anit-virus program. Then, download Media Player again and install it again.