I was trying to install SQL Server 2008 Express edition, and installation wizard was not allowing me to proceed further unless I remove SQL Server 2005 Express tools.
I tried removing SQL Server 2005 Express tools from Uninstall Program window, but still the same error continues. In order to fix this you must remove following registry key from registry.
1) Open Run Menu and type regedit.
2) It will open Registry Editor
3) Now go to following location.
HKEY_LOCAL_MACHINE > Software > Microsoft > Microsoft SQL Server > 90
4) Remove Registry Key 90.
Now try to install SQL Server 2008 and it would work this time.