Windows Server 2008 Server has a lovely feature that stops you adding
.Net Assemblies (or .Dlls in generally) into the GAC - C:\Windows\Assembly even when logged in as Administrator.
Lots of articles on the web discuss turning off the UAC (User Account Control) or changing the Local Policies. Also using the GacUtil.exe that comes with .Net Framework returns "Unknown Error".
The solution is actually quite easy and doesn't need Server changes or restarts:-
Open Command Prompt as Administrator (Right click, Run as Administrator).
Run 'Explorer %windir%\Assembly' (or 'Explorer c:\Windows\Assembly)
then run another:-
Run 'Explorer %windir%\Assembly'
Using one of the explorer windows navigate to where your assemblies are located and drag and drop them into the Windows Assembly folder (like usual)
This method also works when trying to uninstall Assemblies from the GAC.