Note: Removing necessary registry entries might cause inconsistency so backup registry before doing changes.
•1. Uninstall the existing SQL Server and all the components from the add remove program.
•2. Backup the registry . Please read in details the MS link before proceeding.
•3. Delete the following keys using regedit:
?–HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server
?–HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer
•4. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall and delete all the sub-keys referencing SQL Server.
•5. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and delete all the keys referencing SQL Server.
•6. Rename all the SQL Server folders in the computer.
•7. Reboot the machine.
If you dont find this useful or have confusion dont proceed. Open a case with Microsoft and allow them to have a look at your system. with my experience with many failed installation registry keys are left in corrupted state and only solution would be to remove them.But you need expert to fiddle with registry