Instructions
Download and install the "Xbox 360 Accessories Software" from Microsoft's website (see Resources). This is the software that allows your PC to recognize your controller. Download the install file from Microsoft's website and double-click the file to install the software.
Reboot your computer to finish installing the "Xbox 360 Accessories Software" package.
Connect your Xbox 360 controller's charging cable to your computer and to the controller itself. This is the same setup you use when you plug your controller into your console, only instead of plugging it into an Xbox 360 you're plugging it into a USB port on your computer. Windows, thanks to the previously downloaded software, will recognize your Xbox 360 controller. You can now use the device to play "Halo 2" on your PC the same way you would use it to play the same game on the Xbox 360 console.