The biggest enemy to desktop parts in a car environment is heat and power requirements. That said, many people have and still use desktop parts in CarPCs. If space is not an issue, then it's really about finding or building a case that fits in the car.
For power requirements, decide if you really need dual CPUs. If you want to keep the parts you have now, think about underclocking to reduce power. Thinking about changing the HD or DVD to mobile parts that also use less power. Pay special attention to selecting a power supply, efficiency is the name of the game. Start by estimating your power requirements, each component should have a spec showing their voltage and current which you can multiply to get the power in Watts. The key is to pick a PSU that fits the usage and not just the biggest possible which can mean more heat. In the past, people have used DC-to-AC converters and then plugged in regular ATX power supplies. This is very inefficient since you're coverting DC to AC then back to DC. Go with a DC-DC supply, I don't know how much a dual CPU will need but it'll be on the high side of what available out there.
The heat (and cold) issue will require at the minimum very good heatsinks. Even with a fan, if the ambient air in a trunk could 50C already. So moving air will hopefully keep the CPU in operational temperature range. The more heat generating components you have, the more air you want to move. The good thing with some of the mobile parts is that they are rated for a wider temperature operating range.
With cold temperatures, the one thing you can avoid is a slight reduction in LCD performance until it warms up. It's something everyone has to live with in the cold. Mobile HDD will work much better than desktop HDD here for sure since moving parts require lubrication and lubrication can harden when cold.
One other thing I forgot to mention is boot times, once you have a setup running then you'll realize you don't want to wait two minutes for the PC to boot up before you can start listening to your music. There are tons of threads on how to optimize the windows environment to speed up boot time or use hibernate or even switch to linux in some cases.