I got these instructions from
http://www.webtree.ca/windowsxp/repair_xp.htm (go there if you want to see pictures... This is an accurate re-install. Otherwise call MS and see if you can get a free support notice.
How to Repair Install: (also called "In place reinstall")
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 are two approaches. The first one should be used if you can still boot to your Windows Desktop. Simply pop the XP CD into the CD-Rom drive and select Install->Upgrade[recommended]. This will install XP overtop of itself in the same way as upgrading from a previous setup. However if your version is an OEM version then you will not have this option. If you do not see the option to Upgrade[recommended] DO NOT choose New Install as that will either overwrite your current setup completely or give you a dual boot setup.
If you cannot boot to the Desktop or you have an OEM version of XP then the following should be used to complete a Repair Install.
Boot with the Windows XP CD and at the Setup Screen press the Enter Key
You will be taken to the Windows XP Licensing Agreement. After reading the agreement press F8 to proceed.
The next screen gives you the option to do a fresh (clean) install or to "Repair the selected Windows XP installation." Press "R"
Windows XP will copy the necessary files to your Hard Drive to begin the installation and will then reboot. You will see the message that informs you to "Press any key to boot the CD". Do not press any keys this time just wait a few seconds and the Windows Startup Screen will be displayed. Following this you will be greeted by the Windows XP Setup Screens.
When Setup has completed you should find all of your previously installed apps and settings are intact.
How To Determine If XP was Activated?
You installed XP and then can't remember if you activated it. Go to Start->Run and enter the following
oobe/msoobe /a
View Product Key XP - Visit the Keyfinder Homepage by Aleks Ozolins
If you have more than one system running XP you have obviously purchased more than one copy of the Operating System. However you may have forgotten which Product Key you used for which system. It happens. I have 5 systems and have had all of them running XP at times. XP does not store the Product Key in a recognizable format in the registry.
Click this link and select "Open" and your Windows XP Product Key will be displayed
Keyfinder is a freeware utility that retrieves your Product Key. has the options to copy the key to clipboard, save it to a text file, or print it for safekeeping. It works on Windows 95, 98, ME, NT4, 2000, XP, .NET, Office 97, and Office XP. It even has the ability by using Microsoft's own script to change the Windows XP, 98, and ME installation key. See the Microsoft Knowledgebase article here Q328874
If your version of XP came pre-installed on your computer the revealed Product Key could be that of an image used by the manufacturer. If this is the case you will likely be unable to use it to install another version of Windows XP such as one purchased separately from the computer. Find an alternate location for ViewKeyXP if my link is broken.
