Assembly ‘X’ could not be uninstalled because it is required by another application

Remove the keys from following location in registry then you can now uninstall them from the GAC.

Step 1:
Goto - Start - Run -Regedit

Step 2:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Assemblies\Global

Step 3 :
delete the class name as in assembly

Step 4:
Uninstall from assembly