I replaced my old Geforce2 Ti with a new 9600 pro a couple weeks ago (much better!). I also have Windows XP. My installation went surprisingly smoothly after all of the horror stories I'd read in other forums.
Try following these instructions:
First, don't use the CD that came with the card. Download the newest 3.9 catalyst drivers from
http://www.ati.com/support/driver.html and burn them to a CD. Then go to device manager/display adapters and uninstall the current video card (if you've already attempted to install the ati card, it should show the ati card). It should prompt you for a reboot. Select no. Then go into add/remove programs and uninstall any remaining nvidia drivers (hopefully, none) and the current ati drivers. then run the ati catalyst removal tool from ati at
http://www.ati.com/support/drivers/m...lystutils.html
(this will get rid of any remaining old ati drivers). Turn off your computer. Install the new ati card (or leave it in if it's already installed). Boot up windows, it will ask for two device installs (primary and secondary displays), cancel each prompt, then run the ati setup from the cd you created.
If this doesn't work, download and run Drivercleaner at:
http://www.driverheaven.net/cleaner/
Read the readme file for Drivercleaner and follow the instructions. It also works well.
If you have any other questions, let me know.