You basically have 2 options:

You can get an inverter which is the cheaper of the two. You don't need one anywhere near 300W though as someone mentioned. Your laptop should use more than 90W or so. With this setup, you are basically doing an extra conversion though.

Your second option is to just buy a new ac adapter that plugs right into the cigarette lighter. This is the more expensive way to go, but it is a lot "cleaner."

2001 Volvo S40-Perrin MBC@ 11psi, EST intake, 35% tint, still slow