and so are my logs: D:\Program Files\Altiris\Notification Server\Logs.
The logs can be archived and this is located on Settings -> All Settings -> Notification Server -> Notification Server Settings -> Log (Tab)
Is your actual agent install on C:? Typically on every NS install I have ever done the Agent went on the same drive as the install. Now, if you installed the agent later, it's possible it isn't, but the main system logs should still be on the same drive.