Riccardo Papini,

You can find a the information on the guide site at:
http://www.projectpluto.com/rare_q.htm#charon_stub
<http://www.projectpluto.com/rare_q.htm#charon_stub>
A copy of which is below.

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Q: When I try to run the Charon astrometry software, I get a error: "Stub
exec failed, dos4gw.exe: No such file or directory".

A: This is the result of a blunder on my part, but is fairly easy to fix.

DOS4GW.EXE is a file used by Charon and the DOS version of Guide; it is a
DOS extender, allowing these programs to break the old
640K barrier that used to be such a problem in DOS. You can find it in the
WATCOM directory of the Guide CD-ROM. Copy it over to
the Guide directory on your hard drive, and the 'stub exec' will no longer
fail.

Once upon a time, Guide's installation process didn't ask you which flavor
of Guide you wanted (Win 3.1, DOS, Win 95/98/NT). It just
copied all three over, all the time. The result was that DOS4GW.EXE would
always be present, as part of DOS Guide. Obviously, that's no
longer the case, and this problem can crop up.