Storage in a car in hot weather is easy- Put the laptop and anything else that worries you into an icechest. If you feel so inclined, you can even add a few gel packs.
Security for the vehicle and contents while you are away is problematic. Our rule of thumb is to insure well, then leave nothing in the vehicle that we'd hate to lose. A friend who travels a lot, frequently parking his van for long walks along the way, has found another solution. He rents a small storage locker in the nearest town and puts all his valuables in there before parking the van at remote trail heads.
That's not a solution if you're on short driving trips with short hikes, but you wouldn't be carrying the laptop for those. I've got an Epson P2000 that suffices for downloads on hikes, but if weight is really an issue I carry more CF cards than I think I'll need. I'm a newspaper columnist and can't get away from required weekly submissions even though I'm on the road for 2-3 month stretches twice a year. Security for the laptop is easy in spite. When I'm going to be away from web access past a deadline or two, I write several columns to cover the time period and submit them early, then leave the laptop and other valuables in a secure location while I backpack.