Don't worry about battery life...running a laptop, regardless of what is being used, DVD or say...Office, it uses the SAME amount of power...its memory it lacks as you open more and more programs
They make the DC power adapters for laptops, you can get them just about anywhere, though they are expensive to some- $60-80 each. But by having this, you never have to worry about the battery, as it runs right from the outlet in the car/truck.
We luck out as our truck as 4 outlets, so both DD's can plug the laptops in, DH can plug in his cell (my battery last LONGER!

) and then the radar detector is in the last one...we only use that when I drive
The laptop I have right now is a Toshiba... it's a 2.0 Ghz Celeron, 256 ram, has windows XP Professional, DVD/CDR 15" screen, and is one of their lighter models.
DH's laptop is a Compaq - also known as Hewlett packard, I can't remember his specs, however his has a much faster processor than mine, and I think twice the memory. However he uses his with a point of sale system, and runs numerous programs at once.
My laptop was more than his, as he hit rebate time...total price for his after rebates was 1300. The sticker price was just over 2K...mine was bought by my employer, and it was 1599 no rebates offered as it was purchased BY a company..DH purchased his personally for his own business.
Brandy