For a search bar like Google or yahoo, just double click the search bar and a draw-down menu will appear with all the most recent searches. Just hold your mouse over the one that say 'Hentai' and just press the 'Delete' button on your keyboard
If you're using Internet Explorer, next all you have to do is go to 'Tools' and then 'Internet Options'. Then click 'Clear History'. Then click 'Delete cookies'. Then click 'Delete Files'. After that, just press 'Ok' and you're done
I don't know how to delete Cookies and Temporary Internet files on Firefox, however