1. Start typing "netplwiz" (without quotes) in the Windows 8 start screen and hit enter to launch the "User Accounts" control panel (on older versions of Windows, click "User Accounts Control Panel").
2. Enter your password if prompted.
3. In the window that opens, uncheck the box stating "Users must enter a username and passwords to use this computer" and click OK (on older versions of windows, first double-click the account you wish to automatically log in).
4. Enter and reenter the existing account password(s) if you're asked.
That's it. From now on, you will not see the password screen anymore.
However, If you don't want to use this feature anymore and are interested in getting your old Windows 8 password screen back, you can restore it by following the above steps—except the 3rd one where you need to select the check box, which we have de-selected previously.
You can follow the same steps to bypass password screen on previous version of Windows too, not just Windows 8.