To install the utility, doubleclick the msicuu2.exe file you downloaded.
Now run the utility ("Start > All Programs > Windows Install Clean Up"). In the list of programs that appears in CleanUp, select any iTunes entries and click "Remove", as per the following screenshot:
Quit out of CleanUp, restart the PC and try another iTunes install. Does it go through properly this time?
I had the same problem. Don[t bother with the 'Windows Installer Clean-Up' because Microsoft has discontinued it. Instead use 'Fix-it' for the problem. It cleared up the problem and even uninstalled iTunes for me. Go here to use this utility: http://support.microsoft.com/mats/Program_Install_and_Uninstall - hope it works for you!