The nice help desk guy got me up and running once again (and in fairly short order too). There are a lot of perks to to working at Microsoft, not the least of which is getting questions answered (either from tech support or the people who designed the software in the first place).
The solution to my problem was to use the recovery module on the XP install CD to manually copy a new system file over from the CD’s system folder. There may have been an automated method but it wasn’t obvious (not to mention the MCE install CD I had wouldn’t do the trick). There was no way I was going to get it solved on my own except by wiping out Windows and starting fresh.
Now I just need to fix all drivers that managed to break in the repair process… how does replacing that one file cause all the other stuff to break?