Try the screensaver set to never and none too. Here's why. Many screensavers max out the CPU, the CPU thermal limit is hit and it shuts down.
The computer is a Dell B110 running XP home edition. When it is powered up the normal desk top displays OK. If I do not do anything after about 20 minutes the computer powers down.
On the Power Options panel the only Power Scheme shown is Minimal Power Management and all times are listed as Never. There are no other Power Schemes available. They seem to have been deleted.
This is very annoying when I return to start using the computer and it is shut down.