Most install/uninstall problems are due to remnants of older versions and
install/uninstall debris blocking normal install processes. Until you
completely clean your system of any older versions, broken install or
incomplete uninstall debris, you likely won't be able to properly install a
new version or program. Installing a new or update program over a previous
install, if even possible, often leads to dysfunction of the new program.
Vista uninstaller and using the uninstaller that comes with the particular
application does a poor job of removing all parts of a program. Sometimes a
proprietary removal tool can help, but even these will leave blocking files
and registry entries on your system.
You can do a manual uninstall with the help of Windows Installer CleanUp
Utility, but the best way to get rid of all remnants of an older program,
failed download and install, or failed uninstall is using Smarty Uninstaller.
This program will scan your system and list all the programs on your
computer, including "broken" parts of programs. It will then run the
program's uninstaller (if present), and then rescan to list any pieces left
on your system. Clicking Cleanup will then remove all traces of your program
and installation of the new or update program can then proceed normally. You
can access Smarty Uninstaller, a number of proprietary removal tools and
instructions for doing a manual uninstall at the tutorial link below.
For iTunes, you need to completely uninstall iTunes, QuickTime, Apple Mobile
Device Support and Apple Software Update, and then you should be able to do a
clean install of iTunes/QuickTime.
http://www.vistax64.com/tutorials/87249-unable-install-latest-version-program-vista.html