The best way to install the Altiris AClient as part of the base imaging task, is to have the Deployment Server copy the most recent version of the AClient to the local drive, followed by a call from the Windows XP Cmdlines.txt file to run the installation.
This method ensures your clients always have the latest version of the agent and ensures freshly imaged computers still can be properly managed.
Unfortunately the command line options available for the aclient install are not as wide-spread as they should be.
Here's what the cmdlines.txt should look like to make this successful. And yes, the quotes are required:
[Commands] "c:\altiris\aclient\altiris-aclient-6.9.164.X86.exe c:\altiris\aclient\aclient.inp -silent -install -scriptedinstall"