This is just a post of what I had found in the Internet and it works for me (See reference for the actual post). While I knew that there are other ways to reset Administrator password, this is very easy.
Below is a simplified steps.
1. Use a Windows 7 boot disk and boot into repair mode so that you could launch the command line utility
copy c:\windows\system32\sethc.exe c:\
copy c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe4. Reboot the machine
5. Once the machine startup and reach the login page, hit the Shift key 5 times and it will launch an administrator command prompt
6. Use net user command to change the password
net user YourUserName YourNewPassword
7. You should be able to log in now. And of course, you should copying the c:\sethc.exe file back to c:\windows\system32\sethc.exe.