Assuming that this card will perform well with your system, here's what you would do. Get the model # off the card you want to install...you said it's an ATI card so go to:
http://mirror.ati.com/support/driver.html
Fill in the appropriate information and you should be directed to a download page for the correct driver for your system. Download it to your desktop and then turn off the computer and install the video card.
When your reboot, hold down the [F5] key, this will allow you to boot into safe mode and you can open the driver program you downloaded from your desktop and it should install the correct driver. Restart your system and everything should be good.....if not, repost or PM me with actual details and I'm positive it'll get resolved for you.