A commenter on another thread asks:
can i upgrade my mobo to a totally different socket type and still use my oem xp license or will some bloke in india tell me “please buy another copy sir”
The short answer is, “It depends.” If you purchased your OEM copy from a reseller and built your own PC, or if you bought a PC from a white box builder who is a member of Microsoft’s OEM System Builder program, then yes, you can upgrade your motherboard and reinstall your copy of Windows. You may have to activate the new installation over the phone, but you should be able to use your existing CD key.
However, if you purchased your copy of Windows preinstalled on a PC from a so-called “royalty OEM” – i.e., one of the largest 20 PC makers – then it is system-locked, and your CD and license key will only reinstall on the same computer (or one with an identical motherboard and BIOS).
Yet another reason why I recommend smaller system makers and DIY projects over the big names.
Ed Bott 
Well, I had a Dell Dimension 4300, that, for reasons I won’t go into detail about (bios update went awry) was rendered useless. I decided to buy a new case, mobo and memory, and use all the other hardware from my original. Of course, I had one heck of a time getting the old HD to boot up with a new mobo, but I did it. Took me about 4 hours to get XP Pro to boot up. Then I was asked for the validation code. I called Microsoft and explained what was going on. After giving him my license key, he gave me a new code to unlock the OS. Been running fine ever since.
PeteK or anyone,
My problem is in trying to transfer Office 2003 from my old computer to my new one. Obviously, I can’t validate on the new computer until someone at MS flips the switch for me, and presumably, this won’t happen until I remove Office 2003 from my old PC (and revert to Office XP).
Anyway, I’m willing to do all this, but don’t know the simplest way. The MS support site seems useful, but the best I find is contact info telling me I’d have to pay $35 to either talk to or email someone about this terribly simple issue. As PeteK mentions, he was able to resolve his issue by calling MS support. Can that be done without paying money to do it? You’d think it could be, wouldn’t you?
Mark, you should still be able to do the installation, at which point Office will work for up to 50 starts. Then try to activate. If you’re unsuccessful, you’ll be given the option to call a toll-free phone number.
Hope that helps.