SQL Server 2005 Tools makes SQL Server 2008 Express install fail
Open Run Menu and type regedit.
It will open Registry Editor
Now go to following location.

HKEY_LOCAL_MACHINE &gt; Software &gt; Wow6432Node &gt; Microsoft &gt; Microsoft SQL Server &gt; 90

Right click on 90 and click on Delete.
Press Yes to confirm deletion.


Now try to install SQL Server 2008 and it would work this time.