A VB6 program that I created has been installed on a wide range of computers
and networks without any problems. Win 95, Win 98 and XP so far..
Occasionally, however, the function refuses to start on some systems, with
the only response being a critical error: 'Unexpected Error!', without even
trying to run.

Well! I didn't expect it, either!

Re-compiling and re-installing doesn't change anything....