XP automatic logs off

Hello
I have a little problem.
I need 2 access databases to stay open all of the time 24/7, 7 days a week.
At different times during the day a scheduled task runs within access.
But XP logs off itself. we logged in last friday and opened the 2 databases. during the weekend it stayed open. But monday it logged off.
I cant find any reason which causes this log off.
Can anyone give me any probable causes?

thanks