This instruction is provided by the former doom9 member miltiOtis. It is provided as is. Please re-read it carefully to make sure you did every step correctly. You might have skipped the step of deleting the DRM folder (that deletes all your licenses as well).

I would prefer to use the following instruction that is proved to be working:
How to uninstall and roll back Windows Media Player to a previous version (Windows XP)