'AutoComplete' isn't specifically part of the Google page. It's a function of your browser. You can delete your browser's AutoComplete History but you should note that it will delete the history for all websites, not just Google.
You've not stated which browser you're using but I'll guess that it's Internet Explorer. (It's the worst browser there is but, because it comes with Windows, it's still the one which most people use). If so:
Click on the Tools menu.
Click on Internet Options.
Click on the Content tab.
In the Personal information section, click on the AutoComplete button.
Click on the Clear Forms button, and answer Yes to the resulting confirmation.
Chris