I managed to kill my computer's Guest acccount by deleting stuff from its
%appdata% folder. I can't log in as Guest successfully.
How can I reset the whole profile to a clean state?
System Proprieties >> Advanced, Click settings on User Profiles then delete the Guest account.
It's good practice for security reasons to disable (but don't delete) the built in Guest and Admin account. Before you disable them, create another Admin and Guest account. This will solve your problem and add some extra security.
Alt Option 1: Use Windows System Restore and restore to a point before you corrupted the guest account.
Alt Option 2: Open Command Prompt with Admin Privileges. Run this command: