All icons open itunes has something to do with the

Today, i accidently changed the "Open With" funtion for all of my desktop icons to itunes, when i was only trying to fix the itunes icon. Now when i double click on any icon, i.e. Mozilla Firefox (the icon for mozilla is the itunes icon) it opens itunes instead. How do i fix this? Literally every single desktop icon, even Microsoft Word and Microsoft Publisher open Itunes instead of their designated applications. Please help!