This sounds like a problem with the monitor. Try this.
Double check your connections.
Boot into safe mode (F8 at system start up).
Uninstall the display drivers (right click my computer, hardware tab, device manager, expand the display adapters, right click on the display afdapter and select uninstall).
Reboot and let windows xp startup normally.
if necessary Re-install the display drivers from Computer MFG's website.
Reboot after they have installed.