Help,icon issues

Hello, when i open my icons as defult all of my icons change to that defult and open in the same program as my defult and i can't change them back to their original program, this is also the same for my start menu and task bar. (e.g when changeing one icons defult to avg, for exsample they all go avg , or note pad or what ever programe i open the defult as) my computer is windows 7 pro
please help,i've spent all day on this trying to get them to go backto normal, thankyou.