Thanks, Phil. I did figure that out yesterday after looking online. I've been setting up Windows folders so I can see the File Properties (what a pain that is) and deleting the XP data one photo at a time. This is really fun to do on 12,000 photos. Most of it went away but I've got about 50 photos that won't let me change them. I get error codes that point to registry errors according to online sources.
0x80070057
0x88982F72
I haven't messed around with my registry because I don't place much credence in what I've seen so far as to an explanation.
The question, of course, is how the XP data got there in the first place. Once again, based on the nature of it, old 'IPTC' I overwrote with Photo Mechanic still hanging on in there as XPSubject and XPComments, I was guessing the now-defunct MediaDex. You know, our old AFCP friend.
I've been experimenting around and find that Windows File Properties writes to XMP. i.e. if I make a change to a photo in WFP the change will show under XMP using ExifToolGUI, but the change will strip the IPTC out of Photo Mechanic!
This is happening on a Windows 7 system; I can't speak for anything else.