1.From the Start screen, type netplwiz. The command for netplwiz, aka the User Accounts Control Panel, appears in the search results in the left pane. Click that command.
2.In the User Accounts Control Panel, select the account you wish to use to log in automatically.
3.Click off the check-box above the account that says "Users must enter a user name and password to use this computer." Click OK.
4.Enter your password once and then a second time to confirm it. Click OK.
5.Restart Windows. Windows now bypasses the lock screen and the log-in screen to automatically take you to the Start screen.
6.Change your mind? To re-enable the log-in screen, simply return to the User Accounts Control Panel using the steps above and click on the check-box for the "Users must enter a user name and password to use this computer" option.
http://news.cnet.com/8301-10805_3-57457967-75/how-to-bypass-the-windows-8-log-in-screen/