First you need a genuine copy of XP. Start the computer and have it go to
the Desktop. Put the XP CD in the drive and let it go to the setup screen.
Exit. Shutdown and reboot. You should see a message shortly after the
computer restarts that says "press any key to boot cd..." You have 5 seconds
to press a key so just press the space bar or the Enter key. If you've done
it right the CD will boot and you will see a blue screen that tells you some
files are being installed. It takes a few minutes and then you see a screen
that asks if you want to setup Windows or Repair using the Recovery Console.
Press the Enter key. You will see the License Agreement. Press F8 to accept
this agreement. If you have the same version of XP as the copycat you will
be advised that a previous version of Windows has been found. Press the R
key and let the CD install XP overtop of itself. This will make it genuine.
If you don't get the option to Attempt to repair Windows you will see a
screen that asks where you want to install. Follow the instructions to
Delete the partition. Next use the option to Create a partition and finally
format as NTFS (Quick). XP will then begin copying files to the hard drive.
This will take several minutes. When finished it will reboot. Leave the XP
CD in the drive but DO NOT touch any keys. Just let the computer keep
booting. You will see the install screen. Shortly you will have an
opportunity to insert the new Product Key. Install should take about 45
minutes.
See the instructions here
http://www.blackviper.com/Articles/OS/InstallXPPro/installxppro1.htm
One of the problems with seeking help here is that most of the techie types
are incapable of thinking that the word "uninstall" might mean remove. They
think that "uninstall" means one thing and then make you feel small because
you aren't using the "proper" techie terms.