Maybe a Fit PC is good for your needs? It runs from 12v, so it would be perfect for in-car use. As a matter of fact, the company even sells a 12 volt cigarrete lighter plug as an accessory. It also has a mini SD card slot, which you can boot from.
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup