Windows XP problem
This can be very annoying if, for example, you were downloading a large file/movie etc overnight. By default, XP updates itself at 3 am. You then get 5 minutes warning that your PC will be rebooted. Even if you were awake to choose the alternative "reboot later", the nag repeats every 5 minutes.
This blog: discusses problem - in particular see entry Fredrik Sept.16,2006
see: Microsoft Knowledge Base: