In Opera 18, I can no longer set a customised search engine to become the default search engine when using the address bar. Also, somehow the default Google Search in my opera is set to Hong Kong, and would not allow me to change to anther country, despite of the fact that I am in the UK. Any ways that I can try to resolve this?

I tried removing all the library folders and Opera, restarted and reinstall Opera, and somehow all the settings are still in place. What are the files that I should remove in order to completely uninstall Opera, so that I can perform an install from scratch?