What is the native resolution of the monitor? if you don't know then search on the internet using the model number
most probably you have set the wrong resolution. Try changing the resolution from display properties, settings tab in XP
I don't think any monitor has a native resolution of 1600x800, it must be 1600x900.
^ create custom resolution from nvidia control panel
^^
If you can't change your resolution then try Uninstalling your video drivers and re-installing them...That might help...And what is the name and Model of your monitor ???