Im pretty sure I downloaded the 64bit version, but somehow I managed to start iTunes after this and it said I could not connect my iphone because the installed version was not 64bit. Kinda weird cause the download link I clicked said 64bit version. I better try one more time!
judging that you've had a similar error message for 2 very different pieces of software it sounds to me like your windows installer has bugged out (which is a PITA to fix sometimes)
Out of curiosity...why do you need it to backup your iphone? Why not just use icloud? I use icloud for backup and cannot actually remember the last time I used itunes for anything.