In Internet exporer, Choose Tools > Internet options> Content > Autocomplete.
There uncheck all boxes ( Save forms, save passwords etc. ) and click thebuttons saying "clear all passwords" , "Clear all saved forms"
In Firefox, choose tools> options > Privacy . Here, in a tab "Saved forms, unclick the box saying "save form information " (or something like that ) and click Clear now button. Do the same with passwords tab in the Privacy options. Also enable the " Clear all data when closing Firefox " option. That you will see in the bottom of Privacy options tab.
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It might still be possible for an adroit computer user to recover the history from the index.dat files that can not be deleted by you.
Or the cache.
For this , download Ccleaner ( Crap cleaner ) from downloads.com Over a million downloads prove its efficacy. Its free!
http://www.download.com/CCleaner/3000-2144_4-10547048.html?tag=lst-0-1
YOu can also try all in one secret maker(free_, though I don't know how good it is. Other softwares abound, but they might not be free.
Also, if you don't want people to know what filesyou have been reading on AcrobatReader( I like downloading ebooks, and my parents don't like me spoiling my eyes on the comp.) or what movies you have been watching on mediaplayers .( It is difficult to remove such traces from all players) ccleaner can comein handy.
But do go to their forums and download the winapp2.ini file. And unzip it in the folder you installed ccleaner in. That will make sure that most applications' histories are deleted. Otherwise , the standard install comes only with a handful 20- 30 applications' settings only. Even without doing this, all your privacy concerns rearding use of IE, Mozilla, etc. in a multi-user environment will be taken care of)