I'm assuming that you're referring to a Windows password, rather than to a BIOS password. (Both can be set to operate when you first start up your computer). That means that you're seeing a blue screen with the Windows logo on it.
If you see that screen, with one or more usernames on it, see if you can get in as the administrator. (Lots of users password protect their user accounts but forget to put a password on the administrator account). Press Ctrl-Alt-Delete twice, to get to a log-on screen. Enter the username as Administrator and leave the password blank. Click 'OK'. If that gets you in, you can then create a new user account for yourself, and log on via that in the future.
If that doesn't work, this free service has plenty of fans:
http://www.loginrecovery.com/
Chris