Ask the Microsoft Experts
Q: When the Automatic Update feature of Windows XP is set to automatically install critical updates and an update requires a reboot but there is no user at the machine when the reboot occurs, is unsaved user work saved before the machine is rebooted? What is the behavior of Automatic Update if the PC is powered down when the update installation time occurs?
A: If someone is logged on and active, a pop-up window will warn them about the reboot; if the user is an administrator, they will be presented with the option to delay the reboot. If a user is logged on and does not cancel the reboot for any reason (such as the machine is locked, or they have stepped away from the machine), the reboot will occur. This can cause loss of unsaved work. Note that many productivity applications, such as Microsoft Office, have a feature that periodically saves work and can help to reduce the risk of data loss in this situation. If the system is up and no one is logged on, the reboot will occur without any further prompting.
If the PC is powered down, the update installation and reboot will occur at the next scheduled time.