Assuming your system is set to boot from the CD-ROM drive and you have
an actual XP CD as opposed to a recovery CD, boot with the XP
CD in the drive and perform a repair install as outlined below. If the
system isn't set to boot from the CD or you are not sure, you need to
enter the system's BIOS. When you boot the system, the first screen
usually has instructions that if you wish to enter set press a specific
key, when you see that, do so. Then you will have to navigate to the
boot sequence, if the CD-ROM drive is not first line, set it first in
the boot sequence. Save your settings and exit with the XP CD in the
drive. The system will reboot.
NOTE, while a repair install should leave your data files intact, if
something goes wrong during the repair install, you may be forced to
start over and do a clean install of XP. If you don't have your data
backed up, you would lose your data should that eventuality occur.
Boot from the CD. If your system is set to be able to boot from the
CD, it should detect the disk and give a brief message, during the boot
up, if you wish to boot from the CD press any key.
Once you have pressed a key, setup should begin. You will see a
reference asking if you need to load special drivers and another notice
that if you wish to begin the ASR (Automatic Recovery Console) depress
F2. Just let setup run past all of that. It will continue to load
files and drivers.
Then it will bring you to a screen. Eventually, you will come to a
screen with the option to (1) setup Windows or (2) Repair Windows
Installation using the Recovery console.
The first option, to setup Windows is the one you want and requires you
to press enter. When asked, press F8 to accept the end user agreement.
Setup will then search for previous versions of Windows. Upon finding
your version, it will ask if you wish to Repair your current
installation or install fresh. Press R, that will run a repair
installation. From there on, follow the screens.