What should be used to extinguish a computer fire?

CO2 in vapor form, or Halon if you can find it.
Read the directions, both can kill in confined spaces.

If a computer is drawing power from an external source and it’s on fire, the first thing to do is unplug the computer from that source.

Welcome to Fluther.

Assuming it’s a laptop or notebook computer that’s on fire, and also assuming that it’s not “fully involved” (that is, like a Yule log in a fireplace, with flames dancing all over it) and that it has been disconnected from external power as @Brian1946 has suggested, I’d grab a set of potholders or a towel and carry it: outside, if possible, to a fireplace if that’s not an option, or to an oven (and close the door on it) or a bathtub, and flood it with water.

I think your biggest danger with this is going to be smoke production. I haven’t heard of computers that become fully involved with flame, but they can get smoky. That’s why I’d want it outside, or in a place where the smoke could be either vented or contained, and failing that, smothered (safely – smothering with a pillow or blankets could burn down the house).

For electrical (class C) fires, the first step is always to secure power; unplug it! Most electrical fires will self-extinguish if you shut off power before it actually bursts into flame. You can smell the burning insulation melt before it catches.

Laptop batteries are a different story. There really is no way to put them out; the best you can do is keep surrounding material from catching fire.

What jerv said. Also a fire extinguisher.

I think you should consider for CO2 fire extinguisher because it does not leave a residue and stick within easy reach of the fire place.