It sounds like you want to wipe windows xp off the disk and install a Linux distribution. http://www.havetheknowhow.com/
suggests using Ubuntu 14.4 long term support as the distribution. You may prefer a different distro. Whatever linux version will be used, you will have to download the live DVD iso file, burn it to disk, and boot your computer from that.
The Ubuntu installation procedure will do what you want. On approximately the fourth screen, there are three options -- install alongside (the windows OS), wipe the hard disk clean and install Ubuntu as the only OS, and something else. Choose the second option and continue the installation.
A different distro may not be capable of wiping the disk during installation. If you don't care about what is already on the hard drive, you can experiment. Or ask here again after you decide what distro you want to use.