This has been driving me mad for the past couple of days, and I wanted to give the solution a little Google-juice in case someone else encounters the same problem.
If you’re trying to install the 64 bit version of itunes on the 64 bit version of Windows 7 and you’re finding that the installer just won’t run, even if you’re running it as administrator, then try downloading it using something other than Firefox. I don’t know why, but if I download the installer in Firefox then it just silently exits immediately after I run it. If I download it using ie8 then it works fine. Weird.