Changing the motherboard constitutes a new system altogether. Your OEM cd became permanently married to the old system you had (old motherboard). Therefore, it cannot legally be used with any other system even if the original system becomes landfill fodder.
The following quotes are from Microsoft..
In general, OEM software may not be transferred from one system to another system. However, the computer system can certainly be updated with new components without the requirement of a new software license. The only exception to this is the motherboard. If the motherboard is replaced , the computer system is deemed "new" and a new license would be required.
Other PC components may be upgraded, including a hard drive. Though if the hard drive 3 is replaced/upgraded, the operating system must first be removed from the old hard drive. To restate: the operating system is "married" to the computer system on which it is originally installed.
Reply from The Microsoft OEM System Builder Licensing Team to a query I posted 9/11/04 and was answered 9/20/04
Subject: OEM XP and upgrading/replacement mother boards
From: "System Builder Licensing" <email@example.com> Sent: 9/20/2004 5:58:48 AM
Thank you for your post, Michael. Generally, you may upgrade or replace all of the hardware components on your customer's computer and the customer may maintain the license for the original Microsoft OEM operating system software, with the exception of an upgrade or replacement of the motherboard. Unless upgraded or replaced under warranty, if the motherboard is upgraded, then a new computer has been created and the license of new operating system software is required. The original Microsoft OEM operating system software cannot be transferred to another computer. There is no difference between pre installed OEM operating system software and OEM operating system software acquired after the purchase of a PC. Regardless, the OEM operating system must remain with the device that retains the motherboard. To activate Windows XP over the telephone, you can simply call a toll-free number displayed on your screen. A customer service representative will ask for the installation ID number displayed on the same screen, enter that number into a secure database, and return a confirmation ID to you. Once you have typed the confirmation ID, the activation process is complete. We hope this information has been helpful.
Thank you, The Microsoft OEM System Builder Licensing Team
Again, you cannot legally use the key code. You MUST buy a new license.
Edited by Murray S., 02 November 2008 - 02:26 PM.