Are you talking about the google search box, where you type in your query?
If so you can double click in the box and all the search terms will appear in a drop down menu, you can delete them one by one using the keyboard delete key. If there are many, you can,in IE7 click tools menu and internet options, in the dialogue that opens, on the front (general) tab, under browsing history,click delete, and choose web forms and delete them from there.(warning... you will also lose any other form data,like your name and address from website forms.)
In IE6 the option is on the content tab, and autocomplete.(If I remember rightly!)