Very occasionally, during MS-SQL install on some systems, an error is displayed on the MS-SQL "Setup Progress" screen referring to MSXML6 causing the setup to fail.  The description of this and remedy is at the following Microsoft link:

 

http://support.microsoft.com/kb/968749

 

The remedy involved using the Microsoft Install Cleanup utility to uninstall MSXML6 Service Pack 2 due to it being defective, then re-running the MS-SQL installer.