I think it's unlikely that iTunes suddenly opens your MS Word documents, pdf-files and executable programs.
So I assume it only opens your media files (like mp3's). That are settings in Windows. In Windows 7 the function to change it is called "Default programs". Once they are set the way you want it, they should stay that way, even if you update iTunes.
Why do you download iTunes so often?
every time i download itunes it changes all my icons on puter and they all open up as iTunes when I click on them.... help!!!!!!