Activation of Windows XP After Losing Original PC Board & Casing

Try connecting the old hdd to the new motherboard.
if it boots then you can continue working without formatting.

only thing is you have to download the drivers from the board manufacturer website.

Either way I would contact MS.  If you are able to boot and load into windows it will create a long code and you will be able to activate via their automated telephone system, alternatively you can speak to an agent.  

Before going ahead with this, read nobus' article on the subject to be sure that you are still legal.  Microsoft may consider a board failure in a legacy system to be expected failure in the course of time, and not a "defective" product.

It really boils down to the type of XP license you are using. If it is a limited license type (OEM) then the activation process could be negatively effected when it recognizes that there have been hardware changes. There is a limit of allowed hardware changes for these types of licenses. Too many changes are seen as it being a new PC which will void the activation. No matter what you do you will have to contact the Microsoft Activation department. Just explain that there was a power surge in the house and it fried your PC. They will most likely reissue the activation tolerance of the license, resetting it basically. This will however depend upon how long you have been using the license. Best of luck =)

Retrieve the key thats in the system currently.....

ProduKey v1.45 - Recover lost Windows product key (CD-Key) and Office 2003/2007 product key.
http://www.nirsoft.net/utils/product_cd_key_viewer.html

If needed, build an install CD off of the files on the drive if they are present, that way you know they will match up.....

 Create a bootable XP CD from your I386
http://www.howtohaven.com/system/createwindowssetupdisk.shtml

What license type is this?  If it's an OEM sticker based install then you'll need a new license.  See: https://www.experts-exchange.com/A_3721.html
For a retail license just extract the key from the old registry.  If it's part of a volume licensing arrangement carry out a "repair reinstallation" with the VLK media to activate it.

Use:

http://www.winkeyfinder.com/  if you can get the system up and running.

create a bart's PE boot disc, , make sure you load magic jelly bean keyfinder in the installation directory of the bart's pe cd you will be making...then start the computer up with bart pe . this will boot windows environment ,then navigate to the folder where you place the keyfinder, and whaa laa!!

google: Bart's PE and Magical Jelly Bean Keyfinder

To resolve this, I finally located the initial machine chasis that came with the HDD and copied the XP product key from there. Next, after changing to this product key, I called Microsoft to activate without hassles.

Issue has been resolved