Go on another computer, get a blank disc like a CD-R, CD-RW, DVD-R, or DVDRW and download an iso image of Windows 7/8, burn the iso image to the blank disc, put it into the windows xp computer that you want to wipe the hard drive on and boot from the disc. Once the windows 7/8 install screen has come up, click next, click install now, accept the terms, click on custom (advanced), a "Where do you want to install Windows?" screen should come up. when it does, choose the hard drive that xp is installed on, and hit format. If you think that it has been formatted, shut off the computer, turn it back on and boot from the hard drive and see if windows xp still remains. You will know that it doesn't if either nothing happens, or it'll say something like "Operating system not found"
You can proceed with installing windows xp. just make sure you have a product key for it, and a windows xp installation disc. if you dont have a windows xp installation disc, just download an iso image of windows xp and burn it to a blank disc and acquire a product key for it, or just get one legally.