MS SQL Express install issues

Please be gentle.. newbie here !

I'm creating an install program (Using Advanced Installer, but I don't think that's relevant) to install SQL Express. I'm trying to do this as a quiet install with no user intervention.

It's failing with a request for a password on the SQL Server Browser. I cant see how to overcome this with a switch.

This is my current command line :

/IACCEPTSQLSERVERLICENSETERMS
/ACTION=Install
/FEATURES=SQL
/INSTANCENAME=SQLEXPRESS99
/SQLSVCACCOUNT="NT AUTHORITY\Network Service"
/SQLSVCPASSWORD='thepassword'
/TCPENABLED=1

I have to say that I don't understand the "/SQLSVCACCOUNT" switch or why its needed, but that seems to work.

Where it's failing is on the Server Configuration screen with :

SQL Server Browser "NT AUTHORITY\LOCAL SERVICE" , with no password.

Can anyone please point me in the right direction (and hopefully explain so that I can learn more!)

Thank you