I'm guessing you started up the laptop, put in the XP disc, and tried to start the setup process? Then you got a message saying you have a newer OS already installed?
You need to boot off the XP disc and start the install process. You can get into the BIOS, and set the DVD drive as the first boot device, or access the boot menu, and choose to boot off the DVD drive. Then you'll see a message to 'press any key.' Do so at that time, and it'll start the format/install process.
Are you sure you want to wipe out the windows 7 install? Why not just set up a dual boot? It's pretty easy, and you won't lose the windows 7 install that you paid for.