Where is the SQL Express install log?

The SQL Express installer (unlike MSDE) produces a log file by default. There is no need to specify extra parameters when running setup.exe.

The log gets written to:

%ProgramFiles%\Microsoft SQL Server\90\Setup Bootstrap\LOG\Summary.txt

For example, on my machine,

C:\Program Files\Microsoft SQL Server\90\Setup Bootstrap\LOG\Summary.txt

Multiple other log files are written to the same folder, because the SQL Express install actually comprises multiple other installs.