Do u know any tips and tricks for computer?

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I am very much interested in tricks and tips of computer. If u know any tip to keep my pc fast and smooth then please tell me. And I know only some trick like turn off computer in 5 sec, change other person password after they log in xp even though u don't know the password, make funny and dangerous virus by notepad, format hard disk by notepad etc. Do u know any interesting trick?