If you find that the Windows Media Player entry is missing in the Set your Default Programs listing, it’s probably because someone removed access to the program in your computer. Here is how you restore it.
1. Click Start, Default Programs
2. Click Set program access and computer defaults
3. Click Custom to expand the section
4. Scroll down to section Choose a default media player
5. Enable the option Enable access to this program for Windows Media Player
6. Click OK.
WMP is not listed?
If Windows Media Player is not listed in the Set program access and computer defaults list, download wmp-programs-list.zip, unzip and run the enclosed REG file wmp-programs-list.reg. This should restore Windows Media Player to the listing.