Your laptop's USB port supplies only 500mA. So, that is impossible with the USB port. Because even in the ideal conditions, output power cannot be higher than input power. In your case:
Your output power: 12*0.7= 8.4 Watts
Your input power: 5*0.5(max) = 2.5 Watts
With a 95% efficiency of the converter, you need at least an ~8.9 Watt input power.
However, if you have a higher current source with 5V, you can do it. And the way to convert a voltage by increasing it to a higher voltage is done by a "boost converter"*.
A boost converter is a switch-mode power supply and it can be fairly easy to build one by using a Simple Switcher from National Semiconductor.
Here is a design I selected using WEBENCH. It uses LM2585 at 100KHz. So it will be fairly easy for you to make it work. Because higher the frequency, harder it gets to build it.
*Generally, I think. Correct me here if I'm wrong, guys.