I got a hotfix from EPiServer and needed to replace a couple of EPiServer dlls registered in the GAC (Global Assembly Cache).

Replacing the dlls are normally done by simply drag and drop from one folder to the c:\windows\assembly folder. If you try this on a Windows 2008 server, you will get an error message "Access denied 'EPiServer.dll' " Right click to uninstall etc does not work either.

The solution is the gacutil tool! On your development machine, go to C:\Program Files\Microsoft SDKs\Windows\v6.0A\Bin folder and copy the gacutil.exe and gacutil.exe.config file. Place these in the c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727 folder on the Windows 2008 server. Then you run the command: gacutil.exe /i <dll path> /f 
The /i is the install command, and the /f is to force reinstall of an assembly.