Itunes wont download onto my ;aptop

Hello,
I just got a new ipod touch. Im trying to download itunes onto my computer but im having some issues. I downloaded it from the apple website and it works but when i click on the desktop icon to open itunes it says, itunes cannot run because some of its required files are missing. please reinstall itunes.
well i do reinstall it and still nothing.
what do i do??