You cannot use Windows Update to install critical updates on your Exchange 2000 server

When you try to install a critical update on your Microsoft Exchange 2000 Server-based computer, the update may download successfully, but the update may not install successfully. You may also receive an error message that is similar to the following error message:When this issue occurs, only critical updates are affected. Non-critical updates install successfully.

This issue may occur if the Microsoft Exchange Information Store service is running when you install the critical update.

To work around this issue, use one of the following methods:

• Do not use Windows Update to install the critical update. Instead, visit the following Microsoft Web site to install the critical update:
  http://www.microsoft.com/downloads (http://www.microsoft.com/downloads)
• Stop the Microsoft Exchange Information Store service before you install a critical update from Windows Update, and then restart the service after the update is complete.