You can do this by changing the default directory of the Altiris Agent on Package Servers. By default, you're going to find packages in <Drive Letter>:\Program Files\Altiris\Altiris Agent\Package Delivery. You probably are using C:\, but for package servers you may want to use D:\ or a similar drive letter.
You can change this by going to Settings > All Settings, then expanding Settings > Agents/Plug-ins > Altiris Agent > Windows, then selecting Altiris Agent Package
In the right pane, select the Package Servers tab and check the box for 'Package Destination Location on Package Servers', then define a custom path, e.g. D:\Program Files\Altiris\Altiris Agent\Package Delivery.
Does this resolve the issue?