There is some anecdotal evidence that the scripts on the Apple site that are supposed to detect the version of Windows you are running may fail for some users - jumping straight to the "Thank you for downloading ..." page with the installer not having been downloaded. As an alternative, you can go directly to the relevant downloads - like the previous 12.1.0.77 release, there are three different installers available, with direct links to these as follows:
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.