http://windows.microsoft.com/en-us/windows-8/forgot-windows-password
Here's what the link says - and my comments are included:-
What to do if you forget your Windows password
If you've forgotten your Windows password, there are several ways to retrieve or reset it:
If your PC is on a domain, your system administrator must reset your password.
NO I'M NOT ON A DOMAIN - I JUST BOUGHT A COMPUTER THAT I WANT TO USE.
If you're using a Microsoft account, you can reset your password online. For more info, go to the Microsoft account sign-in webpage.
NO I DON'T HAVE A MICROSOFT ACCOUNT - I JUST BOUGHT A COMPUTER THAT I WANT TO USE.
Have someone with an administrator account on the PC sign in and change your password for you. (If you have an administrator account and remember the administrator password, you can do this yourself.)
NO ONE HAS AN ADMINISTRATOR ACCOUNT ON THIS COMPUTER - I JUST BOUGHT IT FOR MY MUM.
If you're using a local account, use your password hint as a reminder.
WINDOWS8 MADE US MODIFY HER USUAL PASSWORD SO MANY TIMES IT IS VIRTUALLY IMPOSSIBLE FOR HER TO RECALL IT.
If you've tried these suggestions and still can't sign in: .
I CAN'T "TRY" ANY OF THESE SUGGESTIONS AS THEY ARE'NT APPLICABLE.
If you have Windows 8, you'll need to reinstall Windows.
HOW DO I REINSTALL WINDOWS? IS THAT WHAT THE "FULL RESET" UNDER THE "ASSIST" BUTTON DOES?
If you have Windows RT, you'll need to contact your PC manufacturer.
I HAVE WINDOWS8 - I ASSUME WINDOWS RT IS SOMETHING DIFFERENT.
For more info about changing or resetting passwords, see Change your password.