Sometimes you need to quickly add a dll to the Global Assembly Cache (GAC) – usually when you need it to run with full trust rather than limited trust in an ASP.net or SharePoint setting. The quickest way to do this from any computer is just to drag and drop the assembly onto c:\windows\assembly which will automatically install it in there. You’ll need to do this from two instances of Windows Explorer for some reason. Open one at c:\windows\assembly, then find the dll in another one and drag and drop it. You’ll need to restart IIS (iisreset at the command line) if it’s for ASP.net or SharePoint.
If the assembly is already in there you’ll need to delete it first – right-click and Uninstall or highlight and press delete, as it won’t automatically overwrite an existing one.