I have a computer that has windows xp, the computer turns on, but it has the log in screen.?

the problem is i dont know the password. Is there i was i can get into the system, or reset the password?
Update: the computer was given to me
Update 2: now i have a new problem. How can i connect to the internet with out having to be connected to a modem, it ia a compaq computer how can i be wireless?
Update 3: it is a compaq presario 900
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