I would really wait around for an alternative to putting an old pc in your car, surely the old pc's use more power than modern ones anyway, and the older the hard drive, the less mp's and the quicker it will get knackered from vibration.
To do it successfully you would need a laptor 2.5" hard drive that handles vibration better which would be expensive.
I would go for the Aiwa CD Unit that quadcells mentioned, should be under £200 here, it's just like your normal aiwa cd head unit with the ability to play mp3's and display their included track info.
I am 19, male, uk and Insurance?, Expensive in the uk for young male drives, never. lol

theres the american teens with their "small" 3 litre runarounds.
Would be run to have a pc in your car I suppose but I would save up, get a CD Writer for a bit under £100 and look out for the aiwa unit.
Corm