Thanks for the reply & suggestion.

Well, I've not got much work done the last 48+ hours, but I've learned a lot of DVD problems trivia! What worked here was described in support.microsoft.com regarding uninstalling certain DVD-related software. Now, I didn't exactly uninstall anything, but I DID upgrade my Adobe Encore from 1.5 to 1.5.1 -- and apparently, that's when my "service" became disabled. What fixed it was going into the registry and deleting the value for a lo-end filter.
After that I came up w/ the classic "reads a CD; won't read a DVD". I won't go into all that except to say that unplugging/replugging the IDE cable had no effect, but swapping DVR-108 drives fixed it (seems my old drive was intermittently not seeing DVRs).
Now. Back to (real) work....