Registry trace utility?

Does anyone know of a good shareware / freeware registry tracer? I want to find out what registry keys a program is trying to look up when I run it, so I can import them from my backup.

(Background is that I recently upgraded my OS from 98 to XP via an OEM install, which of course blew away my registry - as I expected; I'd exported the original into .reg files so I could reimport settings later - XP's files and settings transfer wizard didn't grab the old registry entries to put on my new system, so I either have to import keys or reinstall apps, and I'd rather import keys. I've already imported the obvious branches, but some apps are still complaining, so they must be looking for more obscure keys. I can't just import whole branches because XP doesn't like it, and fair enough since some of them cover system internals.)