Like Sandra said, there is a file called "mergeide" see
This link.
Now Microsoft don't like this work around, because it allows you to use one XP OS on more than one PC. But I dare say they had to offer the fix for Mobo and such like failure reasons. Imagine your in business and you have a Modo or CPU failure. And you need that PC and its data up and running fast. Well, by adding said file to the registary, you can whip the hard drive containing the XP OS out of the faulty machine and install it into any another PC with a different MOBO, CPU, memory or whatever, and it will boot up as normal.
So if you still have the old MOBO/CPU kicking about, you could connect it back up. Boot it up and add the file as instructed, after copying it to a floppy from that MS linked page. Then pop the hard drive back into the new upgrade setup.
PS: Dont be put of by the wording on that Microsoft page,it ain't complicated. All you need to do is scroll to the bottom of that page,and copy past that file to a floppy, thats all you need to do. Installing it is easy, you just click "OK" when it asks if you wish to install it...job done. OK you will have to add the driveres for your new MOBO, but they will be on the Mobo driver disk, so thats no hardship.
I add that file to all XP installs, and I have never had problems swopping said installs from one PC to another. I can Zip and email it if you have problems, just PM me, its only 7kb in size.
Hope it helps.
