Had you tried removing any files, prior to following the directions? Or maybe some of what Darrell had already suggested? If so, I don't know, if the instructions for "Rolling back to a previous version of the Player" would work, or not. It's my understanding, that manually deleting files, might alter the ability to uninstall it. I'm not sure.
If you hadn't tried to remove any WMP11 files, and only followed the instructions, did you get as far as where the "Softdate Update Removal Wizard" appears, after removing Microsoft User-Mode Driver Frameworks Feature Pack 1.0? Also, did you read everything in the article, to include the "Note" on the bottom part, which in part says, "You might not be able to roll back to a previous version of the Player if the hidden folder $NtUninstallwmp11$ is deleted from your computer. Some non-Microsoft programs (such as CCleaner) delete this folder in an attempt to remove unwanted files from your computer. I'm not saying it is the reason, you're having problems. There could be quite a few reasons. I don't know enough about what you've done so far, or possible errors along the way. And for that matter, I don't know that much about WMP11 either.
Sorry I can't be of more help..