When my application tries to display the CHM help file under Vista nothing happens at all, no error message, nothing.
I wrote a little test program to isolate the problem to as few lines of code as possible:
SET HELP TO FULLPATH("plantmaster7.chm")
HELP
MESSAGEBOX("Done")
QUIT
ENDPROC

This code works on my Windows 2000 system, but the help file is not displayed on Vista.
BTW, double clicking on the help file in Vista displays the help file.
Checking the properties of the help file under Vista didn't display an enable button.

Mark