You make a slipstreamed CD or uninstall SP2

Slipstreamed Windows XP CD Using SP2
http://www.theeldergeek.com/slipstreamed_xpsp2_cd.htm

How to use AutoStreamer to Slipstream Windows XP Service Pack 2
http://www.simplyguides.net/guides/using_autostreamer/using_autostreamer.html

Windows XP Service Pack 2 - Direct Download
http://www.microsoft.com/downloads/details.aspx?FamilyId=049C9DBE-3B8E-4F30-8245
-9E368D3CDB5A&displaylang=en
or http://tinyurl.com/5bobl

You may also like to look at nLite ( http://www.nliteos.com/index.html )
and especially RyanVM's Windows XP Post-SP2 Update Pack
(http://www.ryanvm.net/msfn/updatepack.html)

How are you starting the installation? That error message will
be displayed if you start the installation from within your
current Windows XP installation. You need to boot your system
from the CD. To do this you'll need to access the setup program
for your computer's BIOS and change the boot device order to
list the CD drive first. Consult the documentation for you
system for help on configuring your computer's BIOS.

Courtesy of Michael Stevens, MS-MVP
How to Access/Enter Motherboard BIOS
http://www.michaelstevenstech.com/bios_manufacturer.htm

Once your in the system's setup program look for a section
titled Boot. Here you'll want to have your CD drive be listed
as the first boot device.

Once that's set, exit the setup program and hit any key when
you see "To boot from the CD press any key".

Here's some info on repairing your installation.

Courtesy of Michael Stevens, MS-MVP
How to Perform a Windows XP Repair Install
http://www.michaelstevenstech.com/XPrepairinstall.htm

Keep in mind that after the repair install, you'll have to
visit the Windows Update site to install all the patches and
hot available for your system. You can create a slipstreamed
installation CD but you're still going to have to boot from the
CD to get access to the Repair option.

Good luck

Nepatsfan