Been kind of testing this but I'm asleep or away when the downloads complete so I don't see what happens. I Right Click the tray icon and set Tixati to "shutdown system" "when all downloads complete". What happens is my PC will be on later and the downloads have completed. Windows has rebooted and is on without Tixati running. I don't have Tixati set to run at boot.

What version of tixati? It sounds like your computer is restarting after Tixati shuts it down. Why is it restarting? Thats the problem.