You'd probably be better off using a 12v water pump (like the Swiftech MCP600 or the Dangerden DD12V-D4), and watercooling your Carputer. Don't snicker. It's not as far-fetched as it may seem to be at first.
You wouldn't need to power it from the PC's PSU directly either. Just rig up a relay system so that the pump turns on when the PSU turns on.
Of course you'd HAVE to use anti-freeze in it, or come winter your pump and pipes will burst. You won't get the low temps that a regular WC'ed system would get, but since you're not going for max overclock, it's not an issue.
Some of the other benefits are that you wouldn't have to cut open a blow hole in your car's underside either. Just cut open a hole large enough to pass exit and return lines through, and then seal around it. If you use a large enough radiator (like an oil cooler), you could just mount it underneath the car without using any fans. The air moving over it will be enough to cool it off. The only major downside to this is that if you're parked over some really hot asphalt, instead of cooling the water, the radiator will heat it. This should be easily overcome if you mount the radiator in a position other than directly underneath the car
The biggest hurdle would be mounting a waterblock onto the motherboard. For an M10000, you'd have to figure out a way to either convert the existing heatsink into a waterblock, or how to remove it without damaging the processor underneath. Once you got the existing heatsink off, a Koolance CP-200 waterblock would do the trick nicely, and still remain relatively low profile.
