1. Open the Windows Services via Start > Run > services.msc. Look for Automatic Updates, right click and stop it. The message box will coming back again after the system is reboot.
1. Open Windows XP Group Policy via Start > Run > gpedit.msc. Scroll the Local Computer Policy > Computer Configuration > Administrative Templates > Windows Component > Windows Update. At here, you can choose to enable the No auto-restart with logged on users for scheduled automatic updates installations or set time for Re-prompt for restart with scheduled installations.