I actually had to check again if I am not telling something wrong. Actually I have a script which dynamicall changes the "Silent.ini" because I want to be able to provide the SQL Username name dynamically. And yes the username is not the sa if I use a username different then the SA.

So in my test I have created the sql user "altiris_sa" which has both "dbcreator" and "sysadmin" role. And once the script has changed the "silent.ini" shows the line SQLUsername=altiris_sa

Additionally the script checks with the same account if the "express" Database already exists and if yes it asks to delete it before continuing. That's also proves that I have access with the user to the sql server.

Have you tried it once yourself?