Microsoft sells a USB reciever that's needed for the Xbox controller to connect to your PC if it's wireless. If your controller is wired it should work straight from the USB port, the drivers are available from Microsofts website too.
Are there any free programs that will let me use my xbox 360 gamepad with my fps pc games? Im used to the gamepad and have no room to use my mouse.
If you have the USB one, there shouldn't be any problem. If you have the wireless one, you have to buy the 360 wireless receiver to connect it to the PC. You also might have to download the Xbox360 PC Utilities package to get the controller detected right.