You'll need to remember the password, or log on to that machine with an
account with administrative rights.
One possibility - a lot of people never set a password on the
"administrator" account. Hit CTRL-ALT-DEL at the welcome screen to see a
dialogue box which allows you to type in "administrator" as the user
account. Try it without a password.
Otherwise, you're stuffed, unless there are unprotected shares (C$ ?) on
your machine which you can access over the network.