Source: http://feeds.gawker.com/~r/gizmodo/full/~3/lUSPCmXo0cg/how-to-disable-the-new-google-search
Oh you people are never happy. I give you a way to try the new Google Search yesterday, and now you are asking about how to go back to the old one. Fine! Be that way! Here's how:
Yesterday's method only set a cookie in your browser, asking Google to serve a different page layout to you. However, this will affect other Google pages in the wrong way. Googlepedia, for example, renders a very narrow search results page.
To go back, go to your browser preferences and look for the Cookies section—this is generally under Privacy or Security. Now you have three options.
• The brute way: Delete all the cookies.
• The less-brute way: Search for your Google cookies, and delete them all.
• The picky way: Search for your Google cookies and look for this
javascript:void(document.cookie="PREF=ID=20b6e4c2f44943bb:U=4bf292d46faad806:TM=1249677602:LM=1257919388:S=odm0Ys-53ZueXfZG;path=/; domain=.google.com");
and delete it.
Once you are done, go back to Google Search and enjoy the old. [How to Try the New Google Search]