Try "Reports -> Notification Server Management" -> Server -> Resource Reports -> Computers". This report contains primary user name; in most cases this will correspond the logged-on user name. NS stores "Last Logon user" name in the database, but to see it you will need to perform some modifications to report.
Also, you may see "Is managed" flag in the Computers report - you may use this flag to sort the computers with and without Altiris Agents, as "Managed=1" means Altiris Agent is installed.
Thank you,
Alex.