Having computer problems?

okay so my dad did this by accient on his computer i don't no what it is mabye corrupted files or something so what the porblem is that all the filles on the destop and the taskbar and the start list progams they all have the same piture on it its the notepad photo on each of them and the files alll open up on notpar which is like ie avg itunes ect.i trying and puting its defult program version back to itself but when i open and choose a program for example i choose itunes i make it the defult program and then all the programs goon a picture of itunes and all open in itunes what do i do help!!