Find your windows xp cd, restart the computer, press delete or f1 (usually) to go in to the bios, go to the boot menu, set your cd drive as first boot device, come out of the bios, pop your windows cd, reboot then when it says press any key to begin windows setup press a key.If you have sata hard drives makes sure you have an install floppy/flash drive for the sata drivers or it wont see your hard drives (or intall vista or windows 7 then you wont need sata drivers)then press f6 when it asks if you need to install sata drivers, when it asks where you want to install to select your old windows partition and choose delete partition, then choose create partition and set it to use all the unpartioned space, then just reinstall windows.
If you are just worried about whats on the hard drive then doing the above still wont remove everything even after a format or partition delete.What you would need would be a linux boot cd or simple dos boot program that will write 0's and 1's to every sector of your harddrive many times over to make sure nobody could ever get whatever data your worried about.