if MSSQL2005 installation failed at MSXML6 part, here's the solution:
1. go to http://support.microsoft.com/kb/290301 and download Windows Installer CleanUp Utility
2. install Windows Installer CleanUp Utility and open Windows Install Clean Up from start menu
3. look for MSXML 6 Service Pack 2 (KB9544590) from the list then click Remove
4. install MSSQL2005 again, and voilà! that should do it!
solution from: http://support.microsoft.com/kb/968749