That is the whole point of a DC-DC power supply. No inverter needed.
And if you mean directly wiring the PC to the battery, then forget it right now.
The battery does not provide 12v. The power in your car delivers between 6v and 13.8v/14.4v and fluctuates quite a bit. A computer needs the correct voltage to get the correct timings to actually work.
Also a PC does not only take 12v and 5v. It needs 3.3v, and older models need negative voltages too.