Simple way is to find the uninstall key in the registry..

code:
$keyname = "keyname for Altiris"
$uninstall = READVALUE("HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\$keyname","UninstallString")
SHELL $uninstall
This may not do the uninstall silently, but you should be able to check Altiris for info on the uninstall process and add the appropriate info to the uninstall string.

(Someone may already have a polished version of this for Altiris, but this is the basic idea.)

Brian