How to stop unexpected Windows XP reboots
If you use Windows XP Pro, you can use a little-known setting to turn off the auto-reboot feature of Automatic Updates. This way, you can configure Automatic Updates to download and apply patches automatically at night, which is good for your security. But AU won't reboot your PC. Instead, a reminder to reboot will be displayed. If you set AU to apply patches at 3:00 a.m., which is the default, you can reboot first thing in the next morning when you're not in the middle of a project.
The best way to change this setting is using Windows XP Pro's built-in Group Policy Editor (it doesn't exist in XP Home):
Step 1. Click Start, Run, enter gpedit.msc, then click OK.
Step 2. In the window that opens, click the plus signs in the left pane to navigate to the following folder, then select that folder:
Computer Configuration \ Administrative Template \ Windows Components \ Windows Update
Step 3. In the right pane, right-click No auto-restart for scheduled Automatic Updates installations, then click Properties. In the Properties dialog box that appears, select Enabled, then click OK and close the Group Policy Editor window.
You can also delay or lengthen the amount of time between reboot prompts by specifying the amount of time under the “RE-prompt for Restart with Scheduled Installations” setting.