Windows BackUp

How would i set up windows back up to back up all user accounts on a PC no matter who is logged in? example: a pc has 3 users, i want documents and settings backed up for all 3 users at once which is easy, but i want to setup just one back up that would run and back up those 3 users even if a user 4 was logged on that pc when they wherent supposed to be.

I thought i had it figured out since when i made a back up under user1 to back up all 3 users files then logged on under user2 and it was in the scheduled task list but it would still only run under the user it was setup for.