How to run Deus Ex: Human Revolution in windowed mode

Turn V-sync off, usually it's the cause.

There should be a vent that leads to the other end of the hallway behind the turret (where the advanced hacking tutorial is made available). There's also a number of cardboard boxes in the vicinity. Just grab some and stack them up in front of the turret so it cannot see into the hallway. The turret won't see you as long as you stay behind the crates.

After you obstruct its view, you could then return to the other end of the hallway and make your way into the room.