You either know it or you have to reset it.
"What to do if you forget your Windows 8/8.1 password"
http://windows.microsoft.com/en-us/windows/what-do-forget-windows-password#1TC=windows-8
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.
If you're using a Microsoft account, you can reset your password online. For more info, go to the Reset your password webpage.
If you're using a local account, use your password hint as a reminder.
If you've tried these suggestions and still can't sign in:
If you have Windows 8.1, you'll need to reinstall Windows.
If you have Windows RT 8.1, you'll need to contact your PC manufacturer.
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Microsoft forbids any assistance being given in these Forums to help you bypass or "crack" lost or forgotten Passwords.
Here is the Microsoft Information outlining that Policy:
http://answers.microsoft.com/en-us/windows/forum/windows_7-security/keeping-passwords-secure-microsoft-policy-on/39f56ef0-5d68-41ad-9daa-6e6019c25d37