Assuming you can boot into Windows, try tapping the F8 key repeatedly. This will boot in safe mode, which means only the core services will be enabled. From there, you can hopefully install it.

If you've already wiped your drive:

1. If you only have the "Upgrade" version of Windows XP, you'll be forced to reinstall an older version of Windows.
2. If you have the full version (most likely), when you boot, you usually press "Delete" key to enter setup. From there, edit the BIOS to change the boot order so that the CD drive is higher priority than the hard drive. Insert the XP install disk, and reboot. From there, you'll be greeted with the Windows XP installation screen.

This doesnt work? I have tried many times and it will not boot from the cd? I cant press anykeys when told to, Its the same when my computer is restarted and it has to go through the scanning process and gives you time to press a key to cancel it but it wont allow me to press anykey again??

You can't access the BIOS? You usually don't "press anykey", it's usually a specific key (such as F1 or DELETE).

If you can access the BIOS, but can't see the CD drive in the boot sequence, your BIOS may be one of the older ones incabable of booting from CD. In this case, you must use a floppy to start the install.

You don't need to install older version ,just have cd read to put in when asked for proof of ownership ,