Collect and analyze the following logfiles which are created during Windows Setup:
%windir%\panther\setupact.log AND
%windir%\panther\setuperr.log
%windir%\panther\unattendGC\setupact.log AND
%windir%\panther\unattendGC\setuperr.log
The Microsoft TechNet article under "Additional Information" provides detailed information on these logfiles and more log files are available.