If so then, for the future, to enable you re-install Windows 10 you will need, before the free offer expires, to register your devices and their Digital Licences with your Microsoft Account.
If you already log into Windows 10 using your Microsoft Account then this has already happened and fresh installs will use that information when it comes to activating your Windows installation.
But if you haven’t then you need to do so ASAP, the free upgrade ends on the 29th July, 2016.
So if you use a local account to login then you need to go to Settings -> Update & security -> Activation and sign in with your Microsoft Account to associate that device and the Digital Licence
If you then want to auto login to Windows, without entering your password, then do the following:
- Press the Windows key & R, to bring up the run dialogue
netplwiz, and press enter
- Uncheck the “Users must enter a user name and password to use this computer” check box
- Click on “Apply”
- Enter your Windows account password, ignore the local user name displayed and do not enter the old password
- Reboot the PC, and all should be done