Since the demo to The Stanley Parable takes away the achievement you earn before you finish, you might be wondeirng it there is a way to keep it. To retain the achievement, play through the demo as you normally would do. After the part where the game resets for the third time and you are put into a room where you need to move a cup, get ready to enter your Steam overlay. Once you put the mug in the bin, a noise will be played - it is at this time where you should enter the Steam overlay.

Make sure the achievement progress is at 100%. If it is not, you most likely pressed into the overlay too quickly, and you should go back to the game and wait a few more seconds. If the achievement is at 100%, then enter your task manager and force quit/stop the process of the game. To enter this, press CTRL+ALT+DEL on Windows.

After you have ended the program, check Steam to make sure you have the achievement still. You're done!

Note: Now that you have the achievement, you cannot play the demo anymore if you want to retain it. If you play the demo further, you will lose your perfect game and will have to start anew. If you really want to see the last 3 minutes of dialouge after the achievement, you can find it easily on YouTube.