Well, actually, since the original post I have figured out some of what happened -- but not why, exactly.  This might be infomative for other Vista users, because I believe it is Vista, and specifically the UAC's that caused this mess!

UAC's limit access to "system" files, which includes "Program Files".  Since Poser stores all of it's "Runtimes" within the Program Files/e frontier/Poser 7 directory, Windows wont allow user access to modify these files (that is, if you are running User Account Contols)!

So, what happens, whenever I added or changed anything in my Runtime directories from within Poser, the files were stored in an alternate directory: (C:/Users//AppData/Local/VirtualStore/Program Files/e frontier/Poser 7).  Go figure!

And, since this "AppData" folder is considered a "system" folder, it is not "indexed" for searching - thus my search fo the missing PMD turned up nothing even though it was safely where I'd put it.  I finally located this folder by turning of the "indexing" for the search.

Since then I have moved the files back into the regular "Program Files" directory.  And I plan to leave UAC turned off, at least until this compatability issue between Poser and Windows Vista is somehow resolves.

Does anyone know of a way within Poser to set up an alternate location for Runtime folders?  I didn't see a way browsing the preferences...

Anyway - this may not turn out to be a totally unique problem, since it could pose a hazard to anyone using Poser in Vista with UACs enabled.  Hope the information saves someone the headaches it caused me!

BTW, PhilC - thanks for the response -- I hadn't tried a system restore yet - actually hadn't thought to try that so I dunno if it would have worked or not.  Apparently, when a Vista administrator (me) turns off the UAC's, any programs using this alternate directory no longer "sees" that hidden directory!  That is a serious flaw with Vista, IMO.

Paul