If you want to delete an assembly in the GAC that's still in use, you'll get the message:
Assembly 'XXXXXXXXXXX' could not be uninstalled because it is required by other applications.
If you have stopped all services and still don't succeed in removing it, try this little trick.

- Go to Start > Run
- Type regsvr32 -u "C:\Windows\Microsoft.NET\Framework\v1.1.4322\shfusion.dll"
- Browse to C:\Windows\Assembly, the 'GAC-view' has gone
- Open the GAC-folder
- Delete the folder with the name of the assembly you want to remove
- Go to Start > Run
- Type regsvr32 "C:\Windows\Microsoft.NET\Framework\v1.1.4322\shfusion.dll"

Warning: I take no responsability at all, try it at your own risk!