Trouble with downloading iTunes

I seriously need to download iTunes like extremely soon because my phone is slowly dying and cannot be charged and so I need to back up my stuff on t iTunes. I have downloaded iTunes twice onto my computer, but everytime I try to click on it, it tells me Apple Application Support is needed for iTunes...please uninstall iTunes and then reinstall. I have done this and I don't understand what Apple Application Support is and how I get it?????????? PLEASE HELP