Yesterday, on 2 PC's the installation of SQL Server 2005 failed.  The first PC failed with the installation of MSXML6 and on SQL Server VSS Writer.  the second PC failed on SQL Server VSS Writer.  We couldn't find out why, even with the install log.

To solve this we put on the extended log on MSI's.  This is a logging for each step of the MSI and results in a big log file.  It's called voicewarmup. So just set it on before the installation and after the installation, set it back off.  You will find the log file in the TEMP folder under the name msiNNNNN.log.  Now you can walk through this log file and find the solution.

Find more info here : http://msdn.microsoft.com/en-us/library/aa369776(VS.85).aspx and here http://www.pctools.com/guides/registry/detail/1127/