Why do users get a 503 Service Unavailable error after upgrading the Microsoft Windows Server 2000 to 2003?

The error message is the result of the Microsoft Exchange Information Store Service not starting because Oledb32.dll was not updated by the Windows upgrade. You can get the correct version of this file either from another installation of Windows Server 2003 or if you install Windows Server 2003 on another partition and copy the file over.

For more information, refer to https://support.microsoft.com/kb/837285/en-us