emacs won't start interactively

[Joe_Trivers]

O great emacs wizards,

I am trying to install emacs on a new (to me) desktop Windows 10 PC. After 
following the download instructions for Windows, clicking on the runemacs.exe 
icon has no apparent effect. Running emacs from the command prompt with a 
--help argument produces the Usage: line and list of command line options. 
Running emacs as:
 emacs-26.1\bin\emacs --batch -eval (message \"hi there\")
produces the expected string:
  hi there
so apparently command line parsing and elisp execution works. However, running 
just emacs-26.1\bin\emacs returns almost immediately with no output, and so 
does emacs-26.1\bin\emacs -nw.

First I downloaded and unpacked emacs-26.2-i686.zip from a mirror and it didn't 
work. I then tried emacs-26.2-x86_64.zip emacs-26.1-x86_64.zip from the 
ftp.gnu.org site, as well as the no-deps version with emacs-26-x86_64-deps 
unpacked on top. None of them worked. I moved the unpacked tree from 
<me>\Downloads to c:\GNU but that didn't seem to help either.

I installed 26.1-x86_64 on a Windows 10 laptop last year (in c:\GNU) and it 
works just fine. Both PCs are set up by my employer's IT department, presumably 
using the same procedure. I can't figure out what's different on this system. 
Using "run as administrator" doesn't seem to make any difference.

Can someone please advise me what might be wrong, or what I can try next?

/Joe

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