One of the changes that came in with Windows XP SP2 is the emphasis on the fact that you should keep Automatic Updates enabled, and the incessant reminders to reboot.
Unfortunately I don’t know of a way to stop the reminders, but this blog post explains how you can prevent XP restarting your machine automatically, which it does if you don’t tell it not to within 5 minutes. This could be inconvenient at best, and could even lead to loss of work.
This particular method only works on the Professional Edition of XP, so if you have Home Edition you’re on your own, I’m afraid.
(via digg)