How can I close my laptop monitor without it going into Sleep mode?

I just got a Dell Inspiron 1525 with Windows Vista Home Basic on it. Everytime I pull down the monitor to close it, it goes into sleep mode. I want to make it so when I close it it has the same effect as if I just turned off the monitor of my PC: the screen is off, but the computer is still running. How can I do that? Thanks.