I originally found this text on http://blogs.wdevs.com/ColinAngusMackay/archive/2005/10/16/10858.aspx but it now seems to be inactive so I copied the entry from Google's cache to keep it for reference. Thanks to the original author for this tip.
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.
In response to this comment, I'd like to flag up here that he is completely right. I offer this tip as a short delaying tactic so that you can continue your work until the end of the day. At the end of the day you shut down your PC and when you reboot the following morning everything is installed and patched up. This should not be used to delay the installation of patches and security updates indefinitely. As a matter of course you should install security updates as quickly as you can.
In response to the person who emailed and said I was a "MORON" for not mentioning the above doesn't work on the Home Edition of Windows XP: I wouldn't have known. I don't use Windows XP Home edition.