I was recently doing some web part development and wanted to stick the AjaxControlToolkit into the global assembly cache on my vista device. Simple enough right? Just drag and drop and assembly into the c:windowsassembly location. Well, not if you running Vista with UAC turned on. If you are running with the UAC configuration, and try to drag and drop the AjaxControlToolkit into the GAC you will get the infamous access denied message as shown below.
So how do we get our AjaxControlToolkit in the GAC? Well, as most things, there are several ways. The first one, albeit, not the most secure way, would be to turn off UAC is Windows Vista and then try
The second way, and probably the most accepted way, is to run a command prompt as adminsitrator.
After you have the command prompt opened under the administrator account, we can proceed to install the AjaxControlToolkit into the Global Assembly Cache by issuing the following command.
GACUTIL /i c:AjaxControlToolkit.dll
If all goes well you should see something like the following 
BAM!! your done. You now have your ajaxcontroltoolkit installed in the GAC and ready to be used across multiple projects!
Hope this helps and and as Walt Disney says “Around here, we don’t look backwards for very long. We keep moving forward, opening up new doors and doing new things… and curiosity keeps leading us down new paths.”