This case illustrates a common troubleshooting technique for some install/uninstall problems: deleting the Installer registry key.
Way back when I started support Softimage, this was how we fixed problems like “Uninstall says Softimage already uninstalled”, even though that was not true.

In a recent MotionBuilder case, the customer got this error when he tried to uninstall MotionBuilder 2011:

There was an error reading the registry key SOFTWARE\autodesk\motionbuilder\2011//defaultproductkey.

The solution:

1. In the Registry Editor, find this key:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products
2. Search for MotionBuilder in the Data.
3. You should find a DisplayName value under an InstallProperties key.
   

   The full key name will be something like:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\83E7ADBE34330A442ACF348F731E5F87\InstallProperties

4. Export the key (as a backup) and then delete it. For example, on machine, I would delete this key:
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\83E7ADBE34330A442ACF348F731E5F87
   On your machine, the key name may be different than “83E7ADBE34330A442ACF348F731E5F87”
5. Try again to uninstall MotionBuilder.