21 Aug. 2012
My main machine is a Windows XP box and on that one when I double click on a zip file it opens a separate window and displays the contents, which is the .paf file.
Tried the same thing on my Win 7 netbook. It doesn't open a separate window, but replaces the current directory view with the contents of the zip file, which once again is the .paf file.
When I double click on the .paf file inside the zip file, Win 7 gives me the same error message that you received.
But because the .paf file in the zip file is the last backup of your regular .paf file, there's no reason to open it; in fact, you wouldn't want to open it because PAF will then revert to the backup version of the file, and you'll lose your most recent changes. The net effect would be the same as going to the File menu within PAF and clicking on the Restore option.
The backup .paf file in the zip file is your fall-back in case you have a major problem with your main .paf file, such as a match and merge gone horribly wrong, or a gedcom import that had problems. Yes, you could start over from a gedcom file that you created, but unless you create the gedcom on a regular basis, it could well be more out of date than PAF's backup files.
I would not recommend deleting the zip file backups.