1. Check the format of the file you are trying to play. While common file types like MP3, MP4 and WMA are all supported by the Windows Media Player, less common or propriety file types like RM or M4A are not supported and will cause an error message to pop up.
2. Update your Windows Media Player/Operating System. Like any program, the Windows Media Player is constantly being updated to improve efficiency, compatibility and aesthetics. Make sure your computer is set to automatically download updates to the operating system (check the Control Panel's 'Security Center' to find this option). To update the media center itself, simply open the program (while connected to the Internet) and Windows Media Center will automatically check for updates. Alternately, you can check the Windows Media Center website (See Resources) for updates.

3. Check file path. If you receive a 'file not found' error message, check the file path. If you are using an old shortcut, the file may have been moved or deleted. Try finding the file you wish to open on your hard drive and open it directly to get rid of the error message.
4. Close other programs. Windows Media Player requires a lot of memory to run, and if you have a lot of other programs open while you are using Windows Media Player, you may receive a 'critical error' or 'lack of memory' error message. To combat this, close other resource-consuming programs, like Internet browsers.
5. Delete unauthorized add-ons. If you have downloaded unauthorized codecs or skins, they may also be causing Windows Media Player to generate errors. Delete these add-ons to restore functionality.