Launch Background Application at Login window?

Hey all, I'm looking at getting net render to run on system startup on some lab machines so we can render stuff while the computers would normally be at idle. Then when someone logs in, it would stop the render process and let them do their thing. And after they log off, have it start again. Any ideas? I just need to figure out how to get the process to work without being logged into the machine...after that I can do the rest with login scripts and launchd items. Thanks, Nate