Normally, the amount of moisture in cold air is low. However, bring something very cold into an average home and you'll get quite a bit of condensation. There's quite a bit of moisture inside most homes. Imagine that moisture condensing inside your laptop when you bring it in after an evening's viewing... not a pleasant thought.

Thermal cycling from very cold to operating temperature can also cause problems. There's the risk that connectors might loosen, causing sproatic problems that are hard to fix. Even worse is the possibility that expansion and contraction of the circuit board could weaken solder joints or even break a trace. Very bad, very bad indeed.

Do whatever you can to keep your laptop warm outside. Someone suggested a hot water bottle - that seems like a good idea. Perhaps you could put your laptop in a box with a water bottle and some insulation like a blanket to keep the whole system warm.