You get two choices: Restart Now or Restart Later. When you click on Restart Later it will nag you again ten minutes later until you restart the computer.
Stop Nagging Me!
There are two methods that can be used to change this behaviour, one temporary the other permanent.
Stop the update service
On the command line, type:
net stop wuauserv
Control Panel -> Administrative Tools -> Services
Find Windows Update Service and click on it, then click Stop.
Both methods will stop the message appearing, but on only until you restart then its back to normal.
Click on the Start Menu then Run and key in "
gpedit.msc" (without quotes) and navigate to the folders to:
Local Computer Policy ..Computer Configuration ....Administrative templates ......Windows Components ........Windows Update
There are two settings and both will work, so it's your choice. Either enable No auto-restart for schedule Automatic Updates installations or set Re-prompt for restart with scheduled installations to a long time interval, like 4400 minutes.
The default values are "Not configured" in case you want to return back to normal.