To get drivers for your system, go to the website of the manufacturer of your system (Dell, HP, Gateway, etc.) and provide the right info for Support/Drivers. You want the video drivers, but it wouldn't hurt to download all drivers that apply to your system.
If you have a custom/self-built system...go to the website of the motherboard manufacturer....etc.
Under Display Adapters...that's where the video drivers are.
Click that plus-sign in front of Display Adapters...double-click the line item that drops down. Some ATI devices have 2, but I think this only applies to capture cards, not regular video cards/chips.
Click the Driver tab...puish the Uninstall button. Reboot.
When the system reboots, XP may automatically reinstall your video drivers. If it does...see if the problem remains. If it doesn't, follow the directions provided by ATI for installing the video drivers. Those directions should be available at the same site you download the drivers from.