From the first result on Google:
Answer #1:
Quote: "Press the 'Source' button on the bottom edge of the monitor. Then press the 'Menu' button to make the "Not Optimum Mode" message go away temporarily. Then configure the monitor refresh rate to 60 Hz, and apply. (If the message comes back on before you get the refresh rate set to 60 Hz, just repeat the Source/Menu again.) Once set to 60 Hz, the messge should not come back, even if you don't use the 'recommended' screen resolution."
Answer #2:
Quote: "Sounds like you need to temporarily use another monitor to reconfigure your display settings to 1280x1024 at 60hz. What's happened is somehow your computer was set to an incorrect screen resolution or refresh rate and the monitor is telling you that it's not going to display anything until you correct it. The problem is that you can't see your computer to fix the display settings.
You should be able to boot into safe mode in windows XP, that will allow you to use your computer (keep hitting F8 on startup), but you'll need to change your display settings on the normal boot to get your monitor working again, which the best way to do that is to temporarily use another monitor."
