After replacing my laptop, I reinstalled MC12 and imported the latest backup of my library. The drive containing all my music files (a Western Digital 160G external drive) had been re-labeled the G drive after Vista recognized my music drive as volume G instead of as volume F (which was the path to my library folders). Drive G is another backup drive. So I used Vista's own utility to reassign G as F.
Then I tried to play a playlist, but it refused, instead giving this error message: "Media Center encountered errors while trying to play the last several files. Please make sure that the path in your media library points to the right location." I could not manually play tracks by double-clicking on them in their own folders in drive F: moving the cursor over the playlist tracks shows that they are correctly pointing to volume F, and all the files are where they are supposed to be.
I managed to get MC12 to play the playlists correctly twice before the error message re-appeared again, but no luck since then. I have unisntalled and reinstalled MC12 several times to no avail.