You probably have Windows Update set to automatically install updates. After an update has been automatically installed, if a reboot is required, it will display this prompt.
You can open the "Remind me in" list and select "4 hours" to limit how often it reminds you to reboot, but I don't think you can totally eliminate it.
I believe if you stop the process as already suggested by Will.Beninger:
sc stop wuauserv
that the process will at some point, be automatically restarted and you will receive the prompts again.
For me, I set Windows Update to only notify me when updates are available. I get a taskbar notification icon that appears when updates are available and I can choose what updates to apply and when to apply them. That way, I only get the Windows update popup after I have specifically initiated an update.