Please note that a repair install will not rebuild your computer's registry....
Note: Click on any image for a larger view of that particular image.
First make sure that your system BIOS is set to boot from your CD (For most systems you can get into your BIOS by tapping the F1 key, if that fails then try the F2 key or Delete Key. If that doesn’t work, consult your computer’s instruction manual on how to access your systems BIOS)
Insert your XP CD and boot your system (depending on your system you may get a prompt saying press “Y" to boot from your cd, if it does, push Y)
When the 'To Setup Windows XP Now…' Screen comes up. Press ENTER,
Then when the License Agreement comes up Press F8
Then, select your current XP Installation and Press R
From here, the repair installation will begin, follow the instructions on the remaining screens (note, it will look very much like the normal XP installation).
You have now repaired your XP installation and kept your files and Programs intact