I recently had an issue with the Activate Windows now and Change product key links in the System UI on Windows Server 2008 did not work. After some searching, I found the following Microsoft KB article: http://support.microsoft.com/default.aspx?scid=kb;EN-US;929826.
This article describes how to use slmgr -ipk from the cmd prompt to change the product ID. Once this is done, run slui from the cmd prompt to launch the user interface for Windows Activation.
