you can find cheap laptops at walmart,
otherwise..................
get security for your car if you wanna lug a desktop around,
I've seen some enthusiasts on CPUmag use carrying cases that strap around their tower with a hand carrying part at the top, you can then tuck LCD wires onto the side of the case with the straps (custom made one possibly),
then unmount the stand to and LCD monitor, and if you want, you can make brackets for sides of the LCD monitor in which they ends of it will have screw holes, so you can screw them into your MODed case, on the opposite end of the mobo, a side window on the tower may or may not be a good idea depending on how strong things get,
anyway, you want a wireless keyboard and mouse obviously, get a wireless MINI keyboard with extra long batter life, a laptop model perferably, and get one of those mice that don't require you to scroll with it even if it's an optical, there are mice out there that let you use it while in the air, I forgot what it's called, but you can find them at Officemax,
speaking of which, at Officemax you can find car plug ins that offer powerful AC plugins, you want the most expensive one, the logic behind that, you'll be able to suck out more electricity without any PSU disadvantages
a MicroATX tower would probably be the best option, where you can rip out, yes RIP OUT your glove compartment beneath your dashboard and carefully insert your PC there, that way the 'shotgun' person (or even the driver) can activate the onoff switch,
then if you want to get crazy, make a bracket for your CDrom/optical controller beneath your tape player, or CD player deck, you can get extended optical drive cables comin' out of your tower to your deck
alright now we're lookin' if you really want to do this, then you probably don't want to mount you LCD onto your desktop machine or even have the need for straps? so you can get a dual display adapter and put up 14" / 15" LCD displays behind the driver and shotgun seats if you want to get crazy or you can rip out your viewrear mirror and have a DVD screen as your monitor, but the text and graphics will be very sloppy, at like less than 800x600, but you can rewire this to receive visual from even your LPT port
you want a printer, just snap out your top part of the dashboard and install a printer above the glove compartment?
somebody please add to this.............. I'm running out of ideas
edit: hahahaa this is probably the stupid stuff I've ever written. you can get a wireless network and just play outside their house in your damn car! hahahaa holy SH!T
get a VGA lock so no one steals your graphics card