Monitor Will Not Turn Off in through Power Schemes

[Guest Luca]

On both of my Dell desktops, I am having the same problem in

Display Properties/Power Schemes. I set it to turn off monitor after

10 minutes, and "apply". Many times, the next day, when I leave either

computer, the monitor fails to turn off after 10 minutes. If I redo

the setting, it goes off that day, but the problem starts again after

rebooting. Each system has a different brand monitor (Dell, NEC). The

NEC is more problematic. I have searched web for solutions to no

avail. Any suggestions as to solving this? XP is always updated, and

even if I turn off all tasks running in backround, problem persists.