Try to boot into Safe Mode and log on using the built-in administrator account which is not password protected by default. After get back in your system, you can reset any other user password through Control Panel. Remember to give us a feedback when you've resolved your problem.
Actually, in Windows 7, the administrator account is disabled by default. During the initial setup, the end user must pick a username with password. That username is granted administrative rights. Now, you can certainly go in and re-enable the administrator account if you wish, but it is not automatic.
So, if you don't have the account and password, you can try the recovery tools JorgeM mentioned, or do a clean installation.