kia ora, Jack.
with that one, let's try the following procedure.
first, head into your Add/Remove programs and uninstall your QuickTime. if it goes, good. if it doesn't, we'll just attend to it when we attend to itunes.
next, download and install the Windows Installer CleanUp utility. use it to clear any existing itunes and/or QuickTime installation configuration information from the PC. (Launch CleanUp, find any iTunes or QuickTime entries in the List that displays in CleanUp, and remove those iTunes and QuickTime entries.)
Description of the Windows Installer CleanUp Utility
next, use the the following document as a guide for manual removal of leftover itunes and quicktime program files:
Removing iTunes and QuickTime for Windows
now try another itunes install. does it go through properly this time?