2 laptops, can i use the old one as a monitor

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I got a new laptop for Xmas - Toshibo 14" widescreen - and was wondering if I could still use the old one, Dell C840 as a second monitor. The Dell has 1600 x 1200 maximum resolution and so it is preferable for multitabling, but I would like to able to do everything on one computer (i.e. only use the Dell as another monitor)