You don't have it reinstall Windows 7, here are some methods I know:
1. Log on your computer by using built-in administrator account in safe mode, its password is default blank if you never change it. So that you can use it to change all other user password.
2. Use a password reset disk if you have already created.
3. If the methods above are unavailable, you have to reinstall your system, or else you must use a password tool to crack it.
Recommend you to use Windows password rescuer, it is simple and safe, and can recover windows password in several minutes. You can free download here: windowspasswordsrecovery.com and try it free if you need. Follow the steps you can easily recover the password. It is only four steps:
Step 1.Download Windows password Recovery Tool 3.0
Step 2.Burn bootable CD/DVD or USB flash drive
Step 3.BIOS setting, set up your computer boot from CD/DVD or USB
Step 4.Reset Windows password by a few clicks.
Go to Windows password Recovery Tool 3.0 official site, it can solve ur problem.