Power options for monitor

hi, the power options for my monitor (NEC AccuSync90) keep going back to shut off after 20 min., i've changed the power setting to never shut off, but after a couple of hours it changes back to off in 20 min., i've used this monitor with other towers and have had no problems of settings reverting back . i'm running xp pro sp2, all updates have been done. and my tower is AMD athlon 64X2 dual core processor 4200+ , 2.20ghz, 1.87GB of ram . so do i need to modify some registry keys or something so that my monitor always stays on ? thank you for your time.