I thought I had spoken too soon about the usability of the portable app, because Preview Window Zoom seemed to stop working.
I HAVE SOLVED THE MYSTERY.
In each session, the Preview Window MUST BE DOCKED the first time I want to zoom it. After that, I can undock it, and the zoom keyboard command will continue to work.
This behavior holds true on XP PRO, and on Vista.
So I am posting this in case there are other folks having the same challenge. If this is a frequent (or default) behavior, I suggest that a note be added to the User's Manual.
Thanks to all for letting me cry on your shoulders.