It could be that your refresh rate goes too high for the monitor if you set the resolution any lower, if you right click on the dektop, then click properties. Go to the end tab, "Settings", then click the button at the bottom that says, "Advanced". A new window will open up, in that window select the, "Adapter" tab, then there should be a button that says, "List All Modes...". Click that and you'll get another window which allow you to select which resolution, with which colour depth and with which refresh rate you want.
Try selecting something like 800x600, True Colour (32 bit), 60 Hertz. That should definately be in your monitors range of capability.
Just in case all those instructions weren't clear, I've attached a screen shot that should help.