It's ages now since I played this game but, if I recall correctly, you click on the drawer to open it then you drag the notebook to reveal the gun. By left-clicking again on the gun should do the trick. Once you get the gun have a real good look at it! Also the lights on the ceiling!
You will also need a patch for later in the game because clicking on somethings (a wardrobe I think) just ends the game and takes you back to your desktop. I have looked at my saved patches and the file reference is bd_p110.exe You will be able to get this at any game patch site.