If you are talking about Windows XP, log in as administrator, using the
administrator password. You can then reset the passwords of other users.
The administrator password may well be nothing - i.e. blank--if you have no
memory of it. In XP Home Edition, restarting in Safe mode is the way to get
a dialog allowing you to log in as administrator.
If your question is about logging into something else, we don't support that
here, I suspect.