And there are only two options: Restart Now/Restart Later. "Restart Later" means that this stupid thing will ask you again in 10 minutes. Yes, if you're willing to work for the next 4 hours until lunch before rebooting, this means you'll need to answer this question 24 times. Did I mention that the dialog steals the focus?However, you'll still need to reboot your computer to change this setting. If the message is on your screen right now you can get rid of it by stopping the "Automatic Updates" service.
Now, to get rid of it: Start / Run / gpedit.msc / Local Computer Policy / Computer Configuration / Administrative Templates / Windows Components / Windows Update / Re-prompt for restart with scheduled installations
You can configure how often it will nag you (I re-configured it for 720 minutes, which means I'll be asked twice on a work day), or completely disable it.