When I got my laptop at the beginning of the year (Lenovo z570) I could connect a monitor to the HDMI port and have my desktop extended from one to the other, but recently, not sure why, but suddenly stopped letting me do this, only allowing one display output to be used at a time.
The graphics card is a NVidia GeForce GT 540M, and seems to consistently misbehave with any program trying to use it, not sure if its causing the issue or not, but thought I'd put it in there.