Windows XP WPA Reactivation Procedure:
-------------------------------------

1.  Disconnect from the Internet.
2.  Install XP.
3.  Reboot in safe mode.
4.  Run regedit.
5.  Modify the following keys:

HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFTNT\CurrentVersion\
Winlogon 
(Change the Activation Required value to zero)

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion
(Change the RegDone value to 1)

6.  Reboot.

Don Lett was the first to send in this info. It's not a crack 
or a malicious way to foil Windows Product Activation (WPA), 
but rather is a way that legitimate users of XP software can 
avoid having to reactivate the software after a full install:

The trick is to copy and re-use the 
c:\windows\system32\wpa.dbl file.
It's small--- mine is just 13K long--- so it will easily fit 
on a floppy. Or, you can copy it to another safe location on 
your hard drive.  Once that's done, go about your 
reinstallation task normally.  After you've reinstalled XP, 
either boot to a DOS floppy or start in XP's Safe Mode/Command 
Prompt (i.e. hit the F8 key when your system "beeps" during 
the boot process). Then copy your original wpa.dbl from
its safe location back into the c:\windows\system32 folder, 
and then reboot.

Because nothing on your system has changed--- it's the same 
BIOS, CPU, RAM, etc., and because you're installing the same 
copy of XP that was previously installed, the "old" WPA key 
should be accepted as valid, saving you the hassle of re-
Activating the software.