System-wide Hooking to catch Postmessages unreliable?

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Hello, I have a slight problem. I just released some software to a little
over 120 people in our company so far, but had to stop as it seems some
computer systems are not liking my system wide hook I made with one of my
..dll's.

Although the hook (WH_GETMESSAGE) is set properly with no errors on
everyones system, some computers will not update the dll variables in my
shared block of memory. We have been unable to reproduce the problem here on
any of our test machines and operating systems (98, 98SE, 2000, XP).
Everything works fine for us. -- isn't that how it always goes!

Is it possible these problem systems infact do set the hook successfully,
but the system does not give priority to the .dll so that it can update its
postmessage operations? Why is their only problems on about 8% of the
computers that run my software? Any input would be appreciated and I thank
you all in advance.
.

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