system
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I agree with the ‘over the top’ install.
Sometimes the only way to repair XP is to reinstall. You do not have to wipe your partition and start over. Just as with previous versions of Windows you can install over top of an existing setup. This has the advantage of retaining your installed applications, data and settings. You will lose previously saved System Restore Points but System Restore will begin creating new restore points again immediately following the Repair Install. You will need to reinstall SP1a and any Critical Updates from the Windows Update Site. Be aware that a Repair Install will leave your system vulnerable to the Blaster and Welchia worms. Do not go on line until you have enabled XP’s firewall first. Then visit the Windows Update Site to patch your system It is always prudent to backup important data before you make changes to XP.
There’s more information at:
http://www.webtree.ca/windowsxp/repair_xp.htm#How%20to%20Repair%20Windows%20XP%20by%20Installing%20Over%20top%20of%20Existing%20Setup:
The article is about 2/3 down the page.
this will use the current software keys of the registry.
just be sure to choose repair 
w0mbat