from blogs.wdevs.com/ColinAngusMackay

Anyone who is running Windows XP SP2 know what I’m talking about. That stupid, annoying, most ill-designed dialog box ever invented in the history of the computer science that asks “Updating your computer is almost complete. You must restart your computer for the updates to take effect. Do you want to restart your computer now?”
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?

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.

Oh, I almost forgot: this setting is only loaded when Windows starts, so a reboot is needed. If that stupid dialog is on your screen now, just stop the “Automatic Updates” service (but keep it as Automatic, so it gets reloaded on the next start) and you won’t see it again.