If it was originally an OEM installation, the bottom line is.... According to the OEM End Users License Agreement, you could only replace the motherboard with the same model or a revised version of the same model. Anything else is considered by Microsoft to be a new computer, and a new copy of Windows is needed.
I have recently obtained a new computer tower. Seeing as it is bigger, I decided that I would migrate my old desktop components (CPU, Hard disks, CD/DVD drives, and PCI cards) over to the newer and empty tower as it has more room and an added AGP slot (as my old desktop didn't). After completing the migration (and finally finding a working set of peripheral settings) I managed to boot it up properly. However, immediately after booting up, a box saying "Your copy of Windows is not Genuine, would you like to activate now?" popped up. Of course I clicked yes so that I may re-activate my copy of Windows. For whatever reason, it wouldn't load my drivers so I chose phone activation. After spending a few minutes on the Microsoft automatic activation hotline, it tells me that my Installation ID is not valid. I spoke with a representative for several minutes trying to explain my desktop migration, only to find out that I need my product key. Unfortunately, my father got me the old computer from his work, so the Product Key sticker on the side doesn't match the operating system (the sticker is for XP Home). What is even worse, is that the re-installation OEM CD he gave me with the computer isn't in a proper Dell CD sleeve and has a Norton GHOST product key on the back instead of Windows XP Pro.  I tried installing a copy of windows into a virtual machine (I do this as precaution so that I don't accidentally delete important school or work files) only to find out the the product key is no longer valid. Since I cannot login to the desktop anymore (it won't let  me due to my dilemma) I cannot retrieve my product key in any way (to my knowledge) Long story short: I do not have my product key for the install of Windows that I currently have, however I do know for a fact that it was and should still be Genuine.
Windows XP Professional Activation problem.