uninstall SQL 2008

To work around this issue, we suggest you to re-install SQL Server Express, and you can try the following steps:
    1. Uninstall the existing SQL Server 2008 R2 Express and all the components from the control panel.
    2. Backup the registry.
    3. Delete the following keys in 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.
    8. Download and install Microsoft .Net Framework 3.5 SP1 from here.
    9. Download and install Windows Installer 4.5 from here.
    10. Check if the computer has Visual Studio 2008. If so, you need to install Visual Studio 2008 SP1. For more information, please see here.
    11. Download SQL Server 2008 Express with Advanced Services from here and re-install it as suggested in this blog.
 
In addition, on the Server Configuration page, please choose a local administrator or domain user account for all of these SQL Server services, such as administrator or <DomainName>/<UserName>.
 
If this not work, could you please help us to collect the detailed error message and the following error log? It is very useful for our research.
C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\LOG\Summary.txt.
C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Log\<YYYYMMDD_HHMM>\Detail.txt
 
For more information, please visit the following links:
How to: Uninstall an Existing Instance of SQL Server (Setup): http://msdn.microsoft.com/en-us/library/ms143412.aspx
A similar thread: http://social.msdn.microsoft.com/forums/en-us/sqlexpress/thread/B27D77D9-E14C-47FC-8459-7E9529623547