Use some creative jumping techniques to get over the lasers. Jump on top of the machines. At one point, use the steam pipes attached to the wall. You can make it through that room without tripping the turrets.
Just checking, but you took out the gunship, as well? I'm assuming so, since I don't think the medic appears in the lighthouse until you complete that part.
Just as fischmama has said, you should be able to install Steam and login on a second (or third?) computer, as long as you aren't logged on at any of your other machines. Theoretically, you could play on both machines at the same time (?) as long as, after you installed the game, pulled the...