This works for Windows XP Pro & Server 2003 only

• In the Start Menu go to Run type gpedit.msc and click [OK] to open the Group Policy Editor
• 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, select Enabled, and click [OK]
  • Double click on Re-prompt for restart with scheduled installations
    • In the settings window, select Disabled, and click [OK]
• Close Group Policy Editor