The following should help you if ever you forget your windows password.

1. Insert either your Windows 7 installation DVD, or a Windows 7 System Repair disc, into your optical drive and then restart your computer. 
Note: If you don’t have original Windows 7 media and never got around to making a system repair disc, don’t worry. As long as you have access to any other Windows 7 computer (another in your home or a friend’s will work fine).
2. After your computer boots from the disc, click Next on the screen with your language and keyboard choices.
Tip: Don’t see this screen or do you see your typical Windows 7 logon screen? Chances are good that your computer booted from your hard drive (like it normally does) instead of from the disc, which is what you want.
3. Click on the Repair your computer link.
Note: If you booted with a system repair disc instead of a Windows 7 installation disc, you won’t see this link. Just move on to Step 4 below.
4. Wait while your Windows 7 installation is located on your computer.
5. Once your installation is found, take note of the drive letter found in the Location column. Most Windows 7 installations will show D: but yours may be different.
Note: While in Windows, the drive that Windows 7 is installed on is probably labeled as the C: drive. However, when booting from Windows 7 install or repair media, a hidden drive is available that usually isn’t. This drive is given the first available drive letter, probably C:, leaving the next available drive letter, probably D:, for the next drive – the one with Windows 7 installed on it.
6. Select Windows 7 from the Operating System list and then click the Next button.
7. From System Recovery Options, choose Command Prompt.
8. With Command Prompt now open, execute the following two commands, in this order:copy d:\windows\system32\utilman.exe d:\copy d:\windows\system32\cmd.exe d:\windows\system32\utilman.exe
9. To the Overwrite question after executing the second command, answer with Yes.
Important: If the drive that Windows 7 is installed on in your computer is not D: (Step 5), be sure to change all instances of d: in the commands above with the correct drive letter.
10. Remove the disc and then restart your computer.
You can close the Command Prompt window and click Restart but it’s also okay in this situation to restart using your computer’s restart button.
11. Once the Windows 7 logon screen appears, locate the little icon on the bottom-left of the screen that looks like a pie with a square around it and then click it!
Tip: If your normal Windows 7 logon screen did not show up, check to see that you removed the disc you inserted in Step 1. Your computer may continue to boot from this device instead of your hard drive if you don’t remove it.
12. Now that Command Prompt is open, execute the net user command as shown, replacing myusername with whatever your user name is and mypassword with whatever new password you’d like to use:
net user myusername mypassword.So, for example, I would do something like this:
net user Tim 1lov3blueberrie$Tip: If your username has spaces, put double quotes around it when executing net user, as in net user “Tim Fisher” 1lov3blueberrie$.
13. Close the Command Prompt window.
14. Login with your new password!
15. While not required, it would probably be wise to undo the hack that makes this work. If you don’t, you won’t have access to accessibiily features from the Windows 7 login screen.
To reverse the changes you’ve made, repeat Steps 1 through 8 above. When you have access to Command Prompt again, execute the following:
copy d:\utilman.exe d:\windows\system32\utilman.exe
16. Confirm the overwrite and then restart your computer.
Important: Undoing this hack will have no impact on your new password.