I moved from Norton to Webroot, how can I be sure that I got all of Norton removed from my registry?

Hello there, welcome to the Community!
 
Removing every single reference to Norton is almost impossible.. even the standard uninstall procedure tends to fail.  The best way to remove Norton is to use the Norton Removal Tool.  If there are any references remaining after that, chances are they will not be a huge problem, but you can also run the WSA-System Optimizer and/or any other trusted PC cleanup utility of your choice such as CCleaner might also help a bit as well.
 
I hope this helps!