Now you can use this plugin (works with Firefox and IE7) that will automatically disable Google's Personalized Search. If you want do it manualy you must add &pws=0 to the end of the search string.

Google Personalized Search has been developed on late 2005. It remembers your search history and choices you have made by clicking on specify results. Based on that Google now can sort results for your query that should fit to your previous search behaviour.