How can I set up my dual monitors?

Hey everybody, I recently bought a new monitor. Now that I have two plugged into my video card, how can I properly set it up so that I can drag windows between displays? (I have both displays currently working, however I'm stuck working on one, and can't mouse/drag over to the other one)

Windows 7, GeForce 8800GTS 512mb, 19'' Acer/24'' Dell monitors.

Windows Key + P

or right click your desktop > Screen Resolution. Press Detect and Identify.