1) Restart your machine tap the f8 key while its rebooting and a black screen will come up asking how you want to start the computer.
2) Choose start in safe mode
3) Hopefully you will see an account called "Administrator" (you wouldn't see it when you load windows normally).
4) Depending on who set your computer up, the admin account is often not password protected so click and log in
5) One your in, go to the control panel (Start Menu - Control Panel) and click "Users and Accounts"
6) From here you can change or remove your password for the other account. Once you have restart the computer normally and login with your actual account!

However if you do not see the Admin account you will need to try the windows password recovery tool 3.0 to do some password recovery.