Never re-activate after installation
If you have to reinstall Windows XP you normally will have to reactivate too. Well not anymore. Just copy wpa.dbl after you activated the first time. It is located in the Windows\system32 folder. Now if you reinstall Windows XP just copy the file back and you're up and running again
-taken from http://www.activewin.com/winxp/tips/basic/7.shtml
WPA.DBL--The Keeper of the Keys
The WPA.DBL file resides in the Windows\system32 directory and holds the
hardware configuration information and activation state of the current Windows
XP installation. The WPA.DBL file is actually an RC4-encrypted database of the
expiration info of your installation, the confirmation of activation, the
hardware configuration at activation time, and the current hardware
configuration. When you first install Windows XP, this file is approximately 2K
in size--not much more than a stub file. When you activate Windows, this file
grows to approximately 12K-13K, recording the hardware status of your machine.
At each boot, Windows analyzes your current hardware and compares it to the
stored configuration information to see if it has changed. When you make
hardware changes, Windows makes a note of the changes in the WPA file, but keeps
the original configuration for reference. If you make too many changes, Windows
XP will reset the WPA.DBL file back to its original non-activated (2K file size)
state, and you have to reactivate.
As mentioned above, the WPA.DBL file can be backed up to permit activation if
you reload Windows XP. You can also experiment with different hardware
configurations, as we did in preparation for this article. You would back up
WPA.DBL for each configuration change, so you can roll back whenever desired,
similar to what developers may do frequently, as mentioned above. If you save a
copy of the WPA.DBL file at each change of hardware, you can roll back to almost
any state.
One caveat, from our testing--we found that the WPA.DBL was not protected
similar to other system files. If you delete the file, you need to reactivate.
The WPA.DBL is also not included in Windows XP's system restore mechanism.
-taken from http://www.extremetech.com/article2/0,3973,11214,00.asp