If you're installing iTunes on the same laptop then Apple's site will offer the appropriate version of the installer. If you're trying to download an installer for another machine, or are having issues with getting the download to work, use one of the links below:
The third of these - which is actually the 32-bit application with a 64-bit installer - should be used if you get a warning about video card incompatibility when you install the standard 64-bit version, but also may resolve issues with playback, QuickTime compatibility, and problems with some third-party applications (including Outlook).
See turingtest2's notes onTroubleshooting issues with iTunes for Windows updatesfor more details if you're still having problems with installation.