|sounds like the graphics driver went south on you and reset the resolution...|
go to dell site and look for support/drivers for that computer model. there should be a page of drivers specifically for that computer and the xp operating system. Find the video driver and download it to your desktop.
shut down all windows and double click the file and it will reinstall the driver...during this time it will ask you to set the correct video screen(resolution) and you should be ok from there on out!
Change Is Good