In the first room, don't shoot the infected until AFTER they break through the windows and enter the room. It makes the fight more frantic, but if you get solid body shots with the bow, you'll be able to recover arrows from the bodies. Bad arrow shots will usually snap the arrow, but a full draw and solid contact will generally allow you to scrounge up enough arrows to complete the second fight. You should escape the room with 6-8 arrows, then you'll be sitting pretty.