Windows Xp from Dell laptop to Windows 7

Years ago I bought a Dell laptop with Windows XP. It has since died, motherboard failure, and I haven't used it in about a year. I am currently building my own desktop PC. I am also a student and have the optiofn of buying the Windows 7 Home Premium upgrade for 30 bucks from the Ultimate Steal thing. My question is, if I use the existing serial number from the laptop, which is no longer in use, to install windows XP onto the desktop, could I then upgrade to 7? Is it even possible to use that serial to install XP on something other then that laptop? I would prefer to save the 100 or so dollars by using the upgrade vs the OEM full version of home premium.

You can look online for older copies of Windows XP, that come with a COA (Certificate of Authenticity). Just make sure it has never been activated. If you can find one cheap enough, you can use it for the upgrade qualification.

So, even though that laptop no longer works it would still be illegal? What if I didnt want to upgrade and just use XP, would that also be illegal? I guess i dont really understand the legality of the whole thing?
Thanks

its a pity you do not live in Europe because what you propose to do would be totally legal here , a few years ago the EU commission forced Microsoft to change the rules of allowing a defunct pc key to be reused, thats why you can always find old base chassis and ram covers with XP/VISTA key certs for sale on ebay

You can look online for older copies of Windows XP, that come with a COA (Certificate of Authenticity). Just make sure it has never been activated. If you can find one cheap enough, you can use it for the upgrade qualification.

Well that's dumb. I guess ill look for an xp key, thanks!