When you have a computer, you should be aware of what hardware is inside. If not, use something like Speccy to get the hardware for you...although in some cases, software won't detect the card unless it's
drivers are loaded.
If worse comes to worse, open the case and look on the card itself. Then you go to the manufacturer's website and get the
drivers. The card is likely an AMD/ATI or an Nvidia.
In your situation, going to the site for XP
drivers will at least tell you what drivers graphics card you should have.