Try booting in safe mode (press F8 before XP starts to load). It should load the generic XP vga driver. Then uninstall the ATI driver, reboot, and install the new driver. You may want to grab a free copy of Driver Cleaner Pro (http://www.drivercleaner.net/professional.html). Run DCP using the "ATI filter" after you uninstall the old driver and then reboot before installing the new driver. Don't just install the new driver 'on top' of the old one.
If pressing F8 doesn't get you into safe mode, look on the tower to see if all diagnostic LEDS are green. Not certain about your model, but the LEDs may be off instead of green. If there's an error code, look it up in user manual. It's possible the card itself failed, in which case Dell should replace it under the warranty.