The machine in question is offline and I may have 'hatchet jobbed' the 12.04 installation. Ubuntu seems fine so far (how would I know?) and the xp partition is verifiably intact. Somewhere I have seen a warning that boot segments can be moved too 'deep' onto the hard drive to be seen. If THIS is my problem, are they findable/relocatable?
I believe that the answer to my specific problem is found here: http://ubuntuguide.org/wiki/Multiple_OS_Installation#Windows_bootloaders
If reboot to XP was so critical after a partition re-size, I'd have appreciated a warning a little earlier in my learning curve. Maybe it could be added right after "be sure to back up your files."