I uninstalled using the “uninstall utility.” It says that avast was successfully removed, but when I restart the computer, I get messages saying “avast! antivirus is not turned on.”
So, is there still something left of avast! that I can’t find?
I went through Regedit and got rid of anything avast!
You should have used the conventional add remove programs to uninstall avast. The Uninstall Utility is designed to be used only if you have a problem with the add remove programs.
You may have to install avast again, reboot and then use add remove programs.
Can I ask why you uninstalled, perhaps there is something we can help you with?
Welcome to the forums, couchpotato! 
Yes, if you were having a problem, we would all gladly try to help you solve it.