Save Your Windows Activation Key

Microsoft required you to activate(online) your copy of Windows XP(WinXP) after the installation. It is troublesome for some who do not have Internet access, and those always reinstall WinXP.
Microsoft Windows XP required activation
Actually, we can skip the activation process!

However, you need to activate WinXP at least once. :p
After the Windows activation, the Windows Product Activation database files(wpa.dbl and wpa.bak) will be saved into your PC. We can backup the files and then restore later to skip Windows activation.
Backup Windows Activation
Goto the “system32″ folder of your Windows installation folder(normally, c:windowssystem32).
Under “system32″ folder, copy the wpa.dbl and wpa.bak files. The filesize is small, you can save them in a floppy disk.
Done.
Restore Windows Activation
Start Windows XP in Safe Mode(Press F8 key when booting to show windows mode selection menu)
Change directory to the “system32″ folder of Windows installaion(example: c:windowssystem32
)
If exists, rename the wpa.dbl and wpa.bak files in “system32″ folder.
Now, copy the your backup copy of wpa.dbl and wpa.bak into the “system32″ folder.
Reboot and you are done!
Note: The backup only works when you are reinstall Windows XP on the same or similar hardware PC. It does not work for ANOTHER PC for obvious reason.

(Pasted from liewCF.com)