I've had a similar problem. I've used Karen's Registry Ripper, Regcleaner and CCleaner, as well as going manually through the registry with regedit, and deleting all references to Apple (being careful to avoid applets) and to QuickTime and to Quick Time. Nothing works, despite taking my life into my hands this way. I did manage to install Quick Time Alternative, which enables iTunes to work (it's actually by Apple, despite what you might imagine).
I suspect it has as much, or more, to do with Installshield as it has to do with Apple Quicktime. And I did pursue that avenue for a while, fruitlessly - but I've given up now.
Unless there is some guru out there who knows exactly what is happening?
David