If you have a user password set up in Windows clicking on “Windows key +L” on the keyboard will quickly lock your computer but you will not see what is going on behind the screen lock. Clearlock is a free Windows utility to lock Windows while you are away, using a transparent layer that allows you to see what is going on in the background with just a quick look without the need to unlock the computer, ClearLock has a nice 3D like GUI and there is no need to install it.
After entering the wrong password three times there will be a five minutes delay before granting another attempt, the number of invalid password entries are logged, you would be aware of them once you come back to your computer. If you forget your password while the screen is unlocked delete the .ini file inside the program folder to reset it, if the screen is locked you will have to reboot your computer.
This screen locker is a low level protector designed to stop low skilled adversaries, like children, from accessing your computer while you are away, determined serious opponents could plug in a USB thumbdrive in your computer and use Windows autorun feature to run a malicious script to pull your RAM memory and your screenlock password with it or even image the whole hard disk without you knowing.