Control Panel>Programs and Features>click Yahoo Toolbar>click uninstall at top.

You can also choose another default search engine like Google without uninstalling anything. You can have many search engines available. If you have the Yahoo Toolbar installed, you can uninstall that. If you want to remove Yahoo search from the list, use the dropdown arrow on the right of the search box, select Defaults and Remove it from the list. You change the default engine there, too.