If you don't already have the ATI Catalyst Control Center disk for your video card, then you're going to need to download it. At the bottom of the page linked above will be a Download link. This will lead you to a page to select which version you need. You are going to need to know your Operating System and version, ie: Windows XP 32-bit, Windows Vista 64-bit, etc. You are also going to need to know which series your ATI graphics card is in, like 3800, 4800, etc. You can find out our your operating system version by right-clicking My Computer and selecting Properties. To find out your graphics card series, go to "Run.." (Window Key + R is the shortcut) and type in "dxdiag". Go to the Display tab and your graphics card series will be displayed in the first line, ie: Radeon HD 4800 Series.
Once you have selected the right operating system and graphics card series, you will be led to a page to download the ATI CCC. Click "Download" next to Catalyst Software Suite. It should take less than one minute to download over a high speed connection. Once it has finished downloading, run the installer.
On the first screen, Catalyst: Installation Options, click Next. Select the installation location and click Install. When the ATI Software dialog box pops up, click Next. Click Yes on the License Agreement. Select the Express: Recommended installation option. Reboot your computer when prompted to do so.
Once your computer is back up, it is time to configure your graphics settings!