If you have windows go to Control Panel>Programs & Features, then find the doko seach engine and uninstall it. AND/OR you can go to your internet browser settings, and change your default search engine.
Simply uninstalling it via Programs & Features will not help. I recommend you to follow this simple guide http://www.system-tips.net/remove-doko-search-com-doko-search-engine/ In this guide you will see that the articles recommends automatic removal. Well, don't buy this program. However, you may install it and click "Tools" and "Reset browser settings" in it. This will help you get rid of doko search virus for free.