Take a look to see if there's an 'Apple' folder on your computer. If there is, try renaming it, so that Windows no longer recognises it and allows the iTunes installation program to create a completely new folder. Don't worry, it's easier than it sounds!
Click 'Start'
Click 'Computer'
Double-click 'Windows (C:)'
Double-click 'Program files (x86)'
If there's an 'Apple' folder there, right-click on it and select 'Rename'.
Change the name to 'AppleOLD'
Close the window and try again to install iTunes.