Gentlemen, I need help.

I have a html or .chm file for my help file.
In my computer it works great. In a few of my customers, it works Okay.
However, in most of my customers when they click on the menu help file, nothing happens. They can't get to the .chm help files.
Some of them do have windows XP so I'm not sure what the problem is.

My code:

xdirect = SYS(5)+SYS(2003)
DO case
     CASE Which = 1
          ShellExec(xdirect+"\HELPFILE.PDF")
     CASE Which = 2
          ShellExec(xdirect+"\SCGHELP.CHM") 
     CASE Which = 3
          do oldprtinstr
     OTHERWISE 
          DO oldprtinstr
ENDCASE 

As you can see, I have added a choice of the PDF file which always seems to work fine. It is a collection of the html file. It does the job but the html help file would be much nicer and easier to use by the customer.

Because the .chm file always works in my computer, I can't figure out what is wrong.

What am I doing wrong ?
Is there another code besides ShellExec to use ?

Thanks,
Loren