I think the main issue is the value of the laptop hardware - typically at least a thousand dollars, and easily fenced for a few hundred. Obtaining a replacement laptop on short notice in the north country would be difficult, never mind the hassle of installing and configuring all the needed software.
North Conway has at least five banks - call them and see which ones offer safe deposit boxes. A box big enough for a laptop might cost a hundred dollars or so for a month - not sure if they'll bargain for a shorter rental. If not, consider renting a hotel room and hiding the laptop there or leaving it with the front desk.
I think it's pretty safe in the trunk, especially if you are clever about hiding it, but I would rather encourage all hikers to take all valuables out of cars, lest thieves learn to look for them. I've had a car broken into for a roll of quarters at a WMNF trailhead.