You can do it, but you can't. blah blah blah...
Actually, if you are really serious about doing this, and wanna have something that is original by you. Go for it!! I did it once, and I loved it, but I didn't do just the Cd-rom.....I did the whole damn computer!!
All you really need to do this is, a 12V to 120V converter ( hid mine in the glove box), a slim line case ( or equivalent), and a nice lil Bare bones kit from Tiger Direct. ( www.tigerdirect.com) just make sure the motherboard doesn't have on-board audio! you wanna get one of the new sound cards that support 5.1 or 6.1 sorround. ( the best thing about those is that you can make individual adjustments to the channels.). For the monitor, most of the new LCD screens for mobile theater can easily be adapted to a video card.
Assemble the puter as you normally would....but the best part is you save some cash by not having to use the latest Operating System (OS). I'm sure you know someone who has win95 laying around. hide the case somewhere in the car.....or if you're feeling frisky, make your own case ( it sounds weird, but I know a guy who made a computer case completely outta legos!! I wouldn't recommend it for a car, but it goes to show, computer cases, like car audio has no limits!) All you might need is a lil fiberglass and/or plexi. Please note: you can get by with a P1 133 with 32megs of RAM and Win95 and a lil 3.2 GB Hard Drive.....in fact that's all I'd use..it's cheap!! After you got the case mounted and such hook the speakers up to the sound card, or the amplifiers to the sound card...however you're running them.
Viola'!! you can use a CD-Rom drive and have the whole damn puter in your car.....The best part is....Not only can you play cd-roms, you got mp3's. and for those long and annoying stops in traffic, you gotta a video game system (emulators Guys!!! I play PS2 games on my lil machine)....and if you get a DVD-Rom drive you gots the movie part taken care of. Befriend a Computer Programmer, and someone killer in Electronics and car audio (if you yourself aren't any of the above), and the possibilites are endless. Just be picky on the CD and DVD drives....I think Pioneer makes a few for the puter, and they seem to handle up to the bumps.....Yes I have done this, and I love it....Plus I always gotta RTA with me! 
I know this was long, but hopefully I helped some out, and gave ideas to others....Mobile Electronics: 1/4 knowledge, 1/4 theories and laws, 1/2 imagination!!!
LaTeR.
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