First off, thanks for the replies and interest. I also posted to a couple WinXP-specific groups, and got 3-4 revelant replies (and several OT replies re: cross- vs. multiple posts).
After many re-boots using different boot disks, &tc., I was still unable to get the WinXP repair module or re-install to work. After a few attmepts, I finally got Norton's emergency disk to boot; it recognized a fault in the WindXP registry and repaired it (at least it saod so). But still no progress otherwise (re-boot cycle continued).
Then, after several boots from the WinXP disk, it finally cited a "missing or corrupt C://win32...... " file, and allowed me re-format and re-load WinXP. That was Friday night.
Since then, it's been pretty stable, I've had it on since, and re-boot often to test; it's done the re-boot cycle once, and I got 3-beeps once, but otherwise it's OK.
I'm able to read all the partitions and recover her files, and even load the WinXP SP2 and SP3 updates.
I'm still suspicious about it though. I've found some compatible MBs on E-bay for cheap, so I'll probalby swap it out ASAP.
Thanks again for all your input.
-Zz