[quote user="Jerry Wise"]
Any program files or UAC protected location will not work with Pegasus Mail on Vista or later....
Doing things the way Microsoft does or wants will not work with winpm-32 unless UAC is turned off which is a bad idea.
[/quote]
On the contrary, as a matter of observable fact, an installation in Program Files has been working perfectly for me on three machines for several years, and this is with UAC turned on. I make no claim it would be so smooth if I were running PMail in multi-user mode, but that has long since ceased to be of interest to anyone here.
Anyway, I have now found the answer to my own question. The self-extractor fails to execute the installer because I have prevention installed against the Cryptolocker ransomware, the nastiest malware I have seen since such things first arose in the late 1980s.
Such prevention against Cryptolocker typically takes the form of a Group or Local Security Policy that prevents executables being invoked anywhere in the %appdata% and %localappdata% hierarchies. As that is the location the self-extractor chooses (for example, C:\Users\< name of current user>\AppData\Local\Temp\WZSE0.TMP), the self-extractor can unpack its files but cannot execute setup32.exe.
Those of you who frequent this place might care to note this for future reference, as I am not the only person, I hope, who has proper defences against Cryptolocker installed and others may report the same issue.