To install another vid card you first go into the vid properties. Right click on desktop>properties>settings>advanced properties.
Change adapter to standard VGA then uninstall all progs & drivers associated with the video card. Shut down completely. Take out your old card insert the new one. Boot the system, Win XP will say that it has detected a new device etc... If you want to use the generic XP drivers let it install them, if XP does not recognize the card it will set it up as standard VGA. It will want to reboot, let it. Once the system boots up install the drivers that came with the vid card if you want all the bells and whistles that came with the card. Then reboot.