The old threads gave me some links, but it took some work to get the right info. The mostlycreative.com site is dead, so I dragged up an old version from the Internet archive at http://web.archive.org/web/20090423080214/http://www.mostlycreativeworkshop.com/Article11.html
This appears to be based on an excellent Microsoft article titled "You receive a Stop 0x0000007B error after you move the Windows XP system disk to another computer":http://support.microsoft.com/default.aspx?scid=kb;EN-US;314082
As it turned out, the correct *.sys drivers were already on my disk, and all I had to do was the registry merge. The hard part was getting ye olde T43 to limp along far enough to do the registry edit. It, too, was giving me BSODs towards the end of the boot, and even safe mode was locking up. I finally tracked this down to the fact that I had removed the CD drive from the T43 while the disk was out--this shouldn't have mattered in the least, but it was the culprit.
Then I popped the disk into the T42p, got a normal boot, added drivers for video, ethernet, and wlan, and everything was happy. Especially my wife, who got all her precious obsolete programs back and won't be forced to use evil Windows 7.
Thanks for the pointers...