1. Empty your Temp directory and restart
Choose Start > My Computer/Computer (or double-click My Computer on the Desktop).
In My Computer/Computer, open Local Disk C:.
Windows XP and Windows 2000: Open Documents and Settings.
Windows Vista: Open Users.
Double-click the folder that bears your username.
Windows Vista only: If you see an AppData folder, double-click it
and skip to step 12. If you don't see an AppData folder, skip to step
9.
Windows XP only: If you see a Local Settings folder, skip to step
12. If you don't see a Local Settings folder, proceed to the next step.
Windows XP and Windows 2000: Choose Tools > Folder Options.
Windows Vista: Choose Organize > Folder and Search Options.
Select the "Show hȋdden files and folders" button.
Click OK.
Double-click Local Settings or Local folder (If on Windows Vista,
you must first double-click the AppData folder). If Windows needs your
permission to continue, click Continue.
Right-click on the Temp folder and on the shortcut menu, choose Delete.
In the confirmation dialog that appears, click Yes.
If you are unable to delete the Temp folder, close all programs,
especially those in the Notification area (system tray) of the Windows
Task bar shown below and repeat step 10.
Restart your computer.
2. Completely remove iTunes and QuickTime
For Windows XP, follow instructions in Remove and Reinstall iTunes, QuickTime, and other software components for Windows XP.
For Windows Vista, follow instructions in Remove and Reinstall
iTunes, QuickTime, and other software components for Windows Vista.
Note: Songs you purchased from the iTunes Store or imported from
CDs are saved in your My Music folder by default and are not deleted by
removing iTunes.
3. Clean up iTunes installer files on the computer
You can download the Windows Installer Cleanup Utility from
Microsoft, which will clean up any leftover installer files that may be
causing issues with your current installation. To do this:
Click here first, and read the important information about the software.
Click the "Download the Windows Installer Cleanup Utility package now" link on that page to download a file titled msicuu2.exe.
Open the msicuu2.exe file and follow the prompts to install it.
Choose Start > All Programs and then click Windows Install
Cleanup; the Windows Installer Clean Up Utility window appears, listing
software that is currently installed on your computer.
Select iTunes from the list and click Remove.
Click OK in the confirmation dialog that appears.
Repeat steps 5 and 6 if you have multiple iTunes entries listed.
Select QuickTime from the list and click Remove.
Click OK in the confirmation dialog that appears.
Repeat steps 8 and 9 if you have multiple QuickTime entries listed.
Click Exit.
Restart the computer.
4. Install the latest version of iTunes
Source : http://en.kioskea.net/forum/affich-36347-can-t-install-download-itunes
