There are power inverters that you could get. They convert the car's 12 volt DC to 110v AC. I'm trying to figure out a more effeicient way to do this, however, seeing as computers RUN off DC.
As for laptops, you don't really need a converter, or I didn't for an old one. If it's 12 volt DC, you should be able to plug it right into a cigaret lighter or off the battery. Don't try this without researching it though cause I don't know how different newer laptops are to my old 486 ;P