I want to switch to Norton, but AVG won’t uninstall

Hey There!
I am having a challenge working on my Antivirus software.
My computer is running on Windows Vista and I have an AVG 9.0 Antivirus software. I want to uninstall it because I want to switch to Norton, but it won’t let me. I am experiencing setup error whenever I try to uninstall it.
Help!
