here is a review of the codecs that seem faulty and seem to be working with movie maker. Its relatively big review. hope it helps.
http://www.papajohn.org/MM2-Issues-A...nd-Codecs.html
Also,I checked out here.
http://www.windowsmoviemakers.net/Fo...x?PostID=98540
If you scroll to the bottom of the page, he has mentioned that it is a conflict with the software
sound forge. If you have installed it, then, unfortunately, just uninstalling it wouldn't fix the problem as the user had mentioned. You will have to go to recovery manager or how/what he has mentioned.
If nothing worked, please post back with whatever more information you can provide.