Unable to install SQL Server 2008 fails with error code 1603
SQL Server 2008 CTP5 is failed to install on Windows Server 2008 core, have no problem in Windows Server 2003.
So whenever you have a problem in installing any edition of SQL Server it is better to refer to the log file that is used to log all the activities during the setup. In this case you will find more information by looking at [C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG\] directory. The problem occurs because the folders that contain the files that are required during the installation of SQL Server do not have a correct layout. Say if you are trying to install SQL Server from a network share folder or from the media, that might have got corruption or unavailable during this setup.
In this case if you have downloaded the CTP5 from Connect site, ensure it has been extracted fully to the DVD or hard disk share where you want to store. Then ensure the following two folders exists :
These two folders must be under the same level of a folder or the root folder of a drive. The names of these folders must be exactly Servers and Tools. The Servers folder contains all the files that are required to install major SQL Server 2005 components, such as database engine. The Tools folder contains tools components and Books Online for SQL Server.