Install SQL Server Express r2 2008 using a configuration file
Unfortunately, you cannot install SQL Express without Administrator rights on the machine and it's not something you can set in the config file or on the command line. If being able to install without Administrator rights is a requirement, you may want to investigate using SQL Server Compact Edition.

See the following MSDN topics for more information:


Using SQL Server Compact 3.5 (Visual Studio)
How to: Deploy a SQL Server Compact 3.5 Database with an Application
SQL Server Compact 3.5 SP2 Books Online
Local Data Overview
Choosing Between SQL Server 2005 Compact Edition and SQL Server 2005 Express Edition -- A little dated now, but much of it still applies.