Step 1: Illusion Setup
Exit all open windows, and press the Print Screen (PrtSc) button on your keyboard. Now open up paint and press Ctrl+V on an empty canvas. Your screen shot will now pop-up! Save it to anywhere you like with any name.
Next, set the wallpaper to the picture file you just saved. You are now half-way there! Wasn't that easy?
Step 2: Removing The Interface
First, open task manager. Now click the processes tab. Now scroll and find explorer.exe. End this process, and your computer will be locked and un-accessible. The user can still Ctrl+alt+delete to task manager, but from there, they won't know what to do; you will.
To bring the interface back, Open task manager and go to the applications tab and press the "New task..." button. In the box, type explorer.exe and press "ok". The interface will now re-load!
There are ways to prevent task-manager from opening, and instead, asking the user for a password. I am not permitted to share how to prevent Ctrl+alt+delete. If you wish to know how, message me.