In Windows 7 you should find the installer in C:\Users\username\Downloads. It is possible that you've encountered a problem that's been around for several iTunes releases, where nothing actually downloads and you skip straight to the "Thank you for downloading iTunes" page with no indication that anything's gone wrong.
You can access the installer directly, using one of the following links:
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).