If you need to restart the Group Policy Client service, you will find that it's grayed out in the services list, and if you try to restart it from the command line, you get the error "System error 5 has occurred. Access is Denied". The reason you see this behavior is the Group Policy Client service needs System account permissions to be managed. If you want to restart it, you have to restart it as the System account.

You'll need the utility psexec.exe from the Sysinternals Suite. Once you download and extract it, open a command prompt (make sure to Run as Administrator) and change to the directory where psexec.exe is located. Run the following commands:

psexec.exe -s -i cmd.exe

This will launch an interactive window as the system account running cmd.exe. From this window, you can run commands as the system account, instead of your normal user account. Now run this command to stop and start the Group Policy Client service:

net stop gpsvc

net start gpsvc