The disk will check your hard drive to see if windows XP is currently installed on it.After the disk has made its check you should see a screen like the one in the first picture on this page.If you do have your Windows XP product key but you're missing your Windows XP CD, you could argue that, since you did actually purchase XP and you have a valid product key and should be able to legally download a Windows XP CD image from anywhere.I tend to agree with that argument, but Microsoft isn't quite on board.Installing a new operating system will cause all of your data to be lost.**On most home computers, you will want to shut down your station. While the station is restarting, begin pressing the F12 key over and over again. Now that you are in the boot options menu, you will need to use the arrow keys on your keyboard to move over the choice for an Onboard or USB CD-ROM Drive. After you have pressed Enter, most Windows XP disks will prompt you to press any key to boot from the CD (As seen in picture 2).When this appears, you will have only a few seconds to press any key on the keyboard.I usually choose the Space-bar because it is the easiest to find and press.
If you realize that you have data on your hard drive and need to back it up, now would be the time to press F3 and exit the installation program to back it up.You will want to scroll over the C: partition that is current on your hard drive.The next thing you will need to do is click the D key.Windows XP is not distributed online so there isn't a legitimate way to download Windows XP, even from Microsoft.No matter where you find it, any XP download you come across online is likely illegal.You would then burn that ISO image to a CD that you would then use to install Windows XP.