To Lock Screen in Windows 7 you have to press the Windows Key + L to lock it, now. The screen would be locked and password protected in case you have one. In case you don't like shortkeys or you keep forgetting them, there's another option for screen lock.

Simply create a new shortcut to C:\Windows\System32\rundll32.exe user32.dll, LockWorkStation and place it whenever you want.

It will work same as Win + L combination, it use Mouse Click event instead of Keyboard Button press in Win + L Key combination.

"Windows 7 Tips:  Lock  Screen Shortcut"