Hi Lillian,
I posted a download for a machine startup script (non-logon script; runs as part of the machine's startup process when connected to the domain) that checks for and installs the Agent. I do need to update it as I have since tweaked it to properly support 64bit OSes (I think the version I have posted right now will uninstall, then reinstall the Agent at each reboot!). If you'd like to take a look, it is available here:
http://www.symantec.com/connect/downloads/altiris-agent-machine-startup-script