A colleague of mine this morning was saying how his computer every few minutes was prompting him that it would reboot itself in 5 minutes unless he clicked ignore on a baloon... problem was that it kept prompting him every few minutes... he finally succumbed and rebooted.  He then found this article: http://www.aviransplace.com/index.php/archives/2004/12/18/tip-disable-auto-restart-in-windows-update/

Explaining how to disable windows XP Professional edition from ever virally rebooting itself without you being around.  I'm not sure I've ever seen windows prompt me before or not, but I know there have been several annoying times I've left my computer locked with browser windows, IDE's, and other programs just waiting for me to be productive the next morning only to find that windows destroyed all that and rebooted itself!  We no more :)

Reprinting steps here in case link above ever doesn't work:

• In the Start Menu go to Run type “gpedit.msc” and press Enter
• Now a Group Policy editor will open. In this window navigate to: Computer Configuration -> Administrative Template -> Windows Components -> Windows Update
• Double click on No auto-restart for scheduled Automatic Updates installations
• In the settings window Choose Enabled and click OK
• Close Group Policy Editor

Shoddy.