The list of installed codecs is in:

Control Panel / Sounds and Audio Devices / Hardware.

Select "Audio Codecs" or "Video Codecs" from the list, and go to the Properties tab.

If you're lucky, you'll be able to select the offending codec and hit "Remove". It ought to be reinstalled the next time you use WMP to open a file that needs it.

I've not tried this, so I don't know if it'll actually work, and I'm offering no guarantees.