To verify the SQL Server Express instance installed, you could use Microsoft SQL Server 2008 Setup Discovery Report. For more information, please see http://blogs.msdn.com/petersad/archive/2009/11/13/sql-server-2008-discovery-report.aspx

If the Setup Discovery Report doesn’t list the SQL Server Express instance, try the following steps to reinstall it:
1. Make sure you have administrative rights on the computer
2. Uninstall the existing SQL Server 2008 and all the components from the control panel. Remember to backup the user databases before uninstalling SQL Server.
3. Backup the registry.
4. Delete the following keys in regedit:
--HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server
--HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MSSQLServer
5.  Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall  and delete all the sub-keys referencing SQL Server.
6.  Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services and delete all the keys referencing SQL Server.
7.  Rename all the SQL Server folders in the computer.
8.  Download and install Windows Cleanup Utility:
http://support.microsoft.com/?id=290301
Run the utility and remove any SQL Server components that you find.
9. Reboot the machine
10. Download and install Microsoft .Net Framework 3.5 SP1:
http://www.microsoft.com/downloads/details.aspx?FamilyId=AB99342F-5D1A-413D-8319-81DA479AB0D7&displaylang=en
11. Download and install Windows Installer 4.5. (Windows 7 has Windows Installer 4.5)
http://support.microsoft.com/kb/942288
12. Check if the computer has Visual Studio 2008.  If so, you need to install Visual Studio 2008 SP1. For more information, please see http://support.microsoft.com/kb/956139
13. Download  SQL Server 2008 Express Edition Service Pack 1 and install it:
http://www.microsoft.com/downloads/details.aspx?familyid=01AF61E6-2F63-4291-BCAD-FD500F6027FF&displaylang=en
 
If it still fails, please provide us the error messages logged into the setup log files which are located in the %Program files%\Microsoft sql server\100\Setup bootstrap\Log\ folder. For more information about how to View and Read SQL Server Setup Log Files, please see http://msdn.microsoft.com/en-us/library/ms143702.aspx.