Hi,
You can find report
Altiris Agent Basic Inventory (no update N days) report, located in the Altiris Console at: Reports\Notification Server Infrastructure\Agent\Altiris Agent Inventory\Altiris Agent basic inventory (no update N days)
and
Computers Not Reporting Inventory in Last N Days report, located in the Altiris Console at: Reports\Asset and Inventory\Inventory\Windows\Inventory Agent\Inventory Capture\Computers Not Reporting Inventory in Last N days).