Heres some info on Product activation:
http://www.microsoft.com/piracy/activation_faq.mspx
Try this next time Lachie
It allows you to save your activation info for re-installs
(of course there are also imaging options like ghost etc that are available to ghost your barebone system after activation)
Double-click My Computer
Double-click on the "C" drive
Go to the C:\Windows\System32 folder (you may have to click on the link that says "Show The contents of this folder")
Find the files "wpa.dbl" and "wpa.bak" and copy them to a safe location. You can copy them on a floppy drive or burn it onto a CD or DVD.
After you have reinstalled Windows XP on your reformatted hard drive, click "No" when asked if you want to go ahead and go through the activation process
Reboot your computer into SafeMode (you can either press F8 as Windows is booting up to see the Windows Advanced Options menu and select SAFEBOOT_OPTION=Minimal
Double-click My Computer
Double-click on the "C" drive
Go to the C:\Windows\System32 folder (you may have to click on the link that says "Show The contents of this folder")
Find the file "wpa.dbl" and "wpa.bak" (if it exists) and rename them to "wpadbl.new" and "wpabak.new"
Copy your original "wpa.dbl" and "wpa.bak" files from your floppy disk, CD or DVD into the C:\Windows\System32 folder
Restart your system (if you followed the directions in Starting Windows XP in SafeMode you may need to go back into MSCONFIG to turn off booting into SafeMode)
lastly: sit back and sip on your bottle of baileys (very important step)
May the force be with you