Check the specifications in the last few pages of your owner's manual, and you should find a temperature range, or maybe two ranges- one for the temps at which you can operate the unit, and one for temps at which you can store the unit. If you don't have the owner's manual, go to the manufacturer's website- most manufacturers put some or all of the specifications for their computers on their site, so people can check the specs before buying.
What someone said above about condensation is probably correct, since I've read it in lots of places. However, I've never worried about it and never had a problem with it. If it's 20 degrees inside my van when I wake up in the morning, I'll fire up my Espar heater and warm the inside up to 70 degrees in 1 hour, with my laptop opened so it'll warm up also. I'll then power it on and it always runs fine, and I've been doing that for the 3 years I've had the laptop. If you're worried about it, I've read that you should warm up electronic items 15 degrees per hour to avoid problems with condensation. So according to that rule, when I warm my van from 20 to 70 degrees I should wait 4.5 hours before using my laptop, but I don't.