If the older machine had a registered/validated copy of WinXP you won't be able to use it on the newer board since Windows XP is a one machine per license OS. Microsoft is probably OK with upgrading disk drives and so on but not motherboards.
Also most cheaper machines come with a "restore disk" of WinXP, if you lose the hard disk you are usually screwed.
I'd start fresh. Go with a two hard drive system, partition so that your data is separate from WinXP and programs. A guy who goes by
Radified has a good tech blog on building a stable system, using Norton Ghost to image the operating systems and programs on a separate hard disk partition and keeping data safe.
First post for me in a while, I've been in computer breakdown hell but my system didn't lose a bit of data following Radified's advice. I may still buy a new machine and salvage a few parts, but even in 20 months many of my parts are hardly worth salvaging.
~ Rick