try the following procedure.
first, head into your start> control panel >Add/Remove programs and uninstall your QuickTime.
if it goes, good. if it doesn't, we'll just attend to it when we attend
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.
Description of the Windows Installer CleanUp Utility
http://support.microsoft.com/kb/290301/ (Copy ans paste the link in internet explorer)
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 to install itunes again. does it go through properly this time?
If not then also try installing latest windows installer software
(Sometimes an iTunes entry or QuickTime entry doesn't show up in
cleanUp, in conjunction with a missing .msi message, if there's an
older version of the Windows Installer on the PC.)