Well you could try typing "Task" without the quotes in the Search Programs and Files off the Start button. Checks and see if AC has a task scheduled. I couldn't find a way to do it manually.
But, if there were one in there, you could click on it to see all of its properties. It would be easy to edit it or change it so it would run only when idle. I tried to make a task using the Amazing Backup.exe. I don't think the task scheduler coule run the executable and click on the Backup button. There is software that can do that. Or possible write a script.
As an afterthought, when you go to the main computer, open backup, then choose Advanced Settings, then select Change Logon, and select Logon as Local System Account or if set that way log in as a user. I have no idea if this will help. I seriously doubut it. Just some thoughts.