The resolution doesn't depend on the monitor, but on the video card. So you need a driver for that.

Either try the 'driver and utilities' disk that came with your machine or videocard, or download from the site of the manufacturer of that card. If your PC happens to be a Dell also (I can only guess), you probably can download the driver from using your service tag.
If the PC originally came with Vista and you downgraded to XP yourself, some detective work might be needed.