SYMPTOMS
While upgrading a Windows Server 2003 SP1 release candidate, you receive the following error message:
This Service Pack cannot be installed on top of the build currently installed on your computer. Cancel this installation process, uninstall your current build, then re-install this Service Pack.
CAUSE
By default, a Windows Server 2003 SP1 release candidate can not be upgraded to the final SP1. Microsoft advises to uninstall the SP1 release candidate and install the final. In case you have an installation based on a SP1 release candidate integrated CD-ROM, you have problem.
RESOLUTION
A simple registry key is the solutions. Simply change the HKLM\SYSTEM\CurrentControlSet\Control\Windows\CSDReleaseType value from 1 to 0. This registry change allows you to install the SP1 without any prompts.
I have heard, that there is a tool called TweakNT that removes the evaluation flag of an installed Windows Server 2003 system. So, in case you are dealing with a evaluation version, try Google.
Thanks to Vincent van Egmond, Sywan Solutions.