A frustrating error.. and the solution is even more so!
If you get this error during SQL Server 2008 installation, ensure the following :
1. All other windows are closed. That includes IE tabs.
2. The currently active installation window (which shows you the status of the SQL Server installs) is always maximized.
That fixed the problem.
Is it a GUI bug? Beats me… where is that SQL Server 2012 Core installation now…..