Hi, friend, you may do as the followings. I have met the same problem before and I found the efficient way from Web:
1. Click on “Start” and then Control Panel in Windows 7.
2. Click on the User Accounts and Family Safety link.
3. Click on the User Accounts link.
Please note that before you proceed, make sure have some kind of portable media to create a password reset disk on. It means that you will need a flash drive or a blank floppy disk; otherwise you will not be able to create a windows 7 administrator password reset disk on a CD, DVD, or external hard drive.
4. In the task panel on the left, click the Create a password reset disk link. When the Forgotten Password Wizard window appears, click “Next”.
5. In "I want to create a password key disk in the following drive: drop down box", choose the portable media drive to create a Windows 7 password reset disk on. Click “Next” to continue.
6. With the disk or other media still in the drive, enter your current account password in the text box and click Next.
7. Windows 7 now will create a password reset disk on your chosen media. When the progress indicator shows 100% complete, click “Next” and then click “Finish” in the next window. Then remove the flash drive or floppy disk from your computer.
Now, you have a reset disk. Then you can follow the steps to windows 7 password recovery with it.
1. In the Windows 7 log on window, click “Reset Password”.
2. Insert your password reset disk and then click on “Reset password" option to open Password Reset Wizard. Click next button to continue.
3. Select the password reset disk from the drop down box. Click “Next” to proceed to the next step.
4. Enter a new password and then type password again to confirm.
5. Click the next button and then click “Finish” button to close the wizard.
6. Now enter a new password for your user account.
The original source is from: http://www.windowspasswordrecovery.net/support/recover_windows7.html