You are running the program INSTSRV with the current users logon rights, and if they are not a local admin they will have insufficient rights to add this service.I know what you are trying to do 
you want to use SU.EXE to get around NT security for a logon prompt? But in order to install the service suss.exe you need to be an administrator, to be an administrator at logon you need SU.exe...... and around and around it goes.
Give this a shot.
http://www.neosoft.com/~brycel/kix/other_scripts/
and take a look at the suss_installer script.
Use the computer list script to create a list of all NT computers on the domain.
Bryce
