While Windows 8 Pro and Enterprise users can use the local Group Policy editor to enable or disable the Lock Screen, everyone else will need to use the Registry Editor.
From the Start Screen type regedit and then click on the regedit icon displayed.
In the User Account Control window click the Yes button to continue.
Browse to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows and create a new key called Personalization (if it does not already exist).
In the Personalization key create a new DWORD called NoLockScreen and change its value to 1 to disable the Lock Screen. Conversely changing the DWORD to 0 will enable the Lock Screen again.