Get a gun at the beginning of the level:

At the beginning of the level where you 
reach the security guard, find a chair. 
It is easily moved. Move it to the door 
that he will open for you, and get him 
to open it. Slide the chair in between 
the doors, and it will not close. This 
may take a few tries. Go out, take the 
crowbar, charge it up until you reach 
the same security guard, and hit him 
until he explodes. Then, get his pistol 
with the laser sight and continue the
game.