What to do if you forget your Windows password
If you've forgotten your Windows password and you're on a domain, you should contact your system administrator to reset your password. If you're not on a domain (most home users are not on a domain), you can reset your password by using a password reset disk or by using an administrator account.
Create a password reset disk:
http://windows.microsoft.com/en-us/windows7/Create-a-password-reset-disk
Reset your Windows password:
http://windows.microsoft.com/en-US/windows7/Reset-your-Windows-password
If you forget the administrator password and don't have a password reset disk or another administrator account, you won't be able to reset the password. If there are no other user accounts on the computer, you won't be able to log on to Windows and will need to re-install Windows.
What to do if you forget your Windows password:
http://windows.microsoft.com/en-US/windows7/What-to-do-if-you-forget-your-Windows-password
Solutions to common problems with logging on to Windows:
http://windows.microsoft.com/en-US/Windows7/Solutions-to-common-problems-with-logging-on-to-Windows
Change your Windows password:
http://windows.microsoft.com/en-us/windows7/Change-your-Windows-password
Microsoft policy about lost or forgotten passwords:
http://support.microsoft.com/kb/189126
Keeping Passwords Secure- Microsoft Policy on Getting Around Passwords:
If you bought Windows and lost the disk:
Reinstall Windows Preinstalled On Computer
OEM computer manufacturers are required to provide at least one way for you to reinstall Windows if you buy a computer with Windows preinstalled - your computer should have included either a restore partition on your hard drive, or Windows install/restore disks.
If the restore partition is still there and was not removed, it will reinstall Windows 7. There is usually a prompt when you first start the computer telling you what key to press to start the restore - check your manual if you're not sure how to. This will remove any personal files on the computer, back them up first.
If you don't have the restore partition:
Contact the OEM computer manufacturer and ask them to send you an install disk for your computer - they may or may not do it. If they agree to send you one they may charge a small fee.