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

Chances are that your assembly has a pin in the registry which stops it from being uninstalled. You can find these here:
* HKEY_CURRENT_USER\Software\Microsoft\Installer\Assemblies\Global
* HKEY_LOCAL_MACHINE\Software\Classes\Installer\Assemblies\Global

Just remove the keys, and you can now uninstall them from the GAC.

Registry editing has been disabled by your administrator

If you get this message when trying to run regedit. Download the vb script and run it.

http://praveenkumarg.blogspot.com/2008/10/registry-editing-has-been-disabled-by.html