continiously customers of my product have trouble to install quicktime correctly on their windows systems (from XP to

This is a real pain since years, especially because my customers complain that my software is garbage because they don't want to differentiate between QT and my software.
In the years I tried all usual tricks, like installing "as admin", switching off the antivir, rebooting, etc. etc. In most cases I succeeded in deinstalling QT and reinstalling QT up to 4-5 times. Sometimes with the same version, sometimes with different versions, but up to now I couldn't find any pattern, what at last lead to success.
I know, that QT is using the old multimedia API on windows and is running out of time and there won't be probably any update on QT anymore, but what would help me a lot for my customer satisfaction is:
1. What exactly is LC checking for when asking for qtversion? a file? a registry entry? This would perhaps help me to track down the problem
2. I can't beleive that I, resp. my customers are the only ones having trouble to install QT on windows, so what are your tricks to succeed in installing QT, so that LC likes it?
Thanks for sharing your experiences
Tiemo