from doing a little more research on this, i think a basic pc powerful enough to run dvd/mp3 could be build from around £250 (excluding the AV adaptor).... the power supply seems to be the main cost, the rest can be second hand parts from ebay which go for next to nothing as they are not ideal for main computer use anymore but more than adequate for an in-car pc. obviosuly the machine could be as powerful or as basic as you required.
i have made a list of the stuff i need to get when xmas is over, so i'd thought i'd list it here in case it helps anyone else interested in doing this.....
assuming you have comand installed for the screen....
video adaptor from
HERE
in-car power supply from
HERE
ultra thin mini case from
HERE
micro atx motherboard something like
THIS
cpu something like
THIS
videocard supporting TV out:
HERE
if the videocard only supports svideo ouput get a converter
HERE
laptop hard drive (and IDE to laptop converter lead)
HERE
tiny keyboard to fit in glove compartment
HERE
USB hub to go in glove compartment
HERE
+ trackball, 512meg ram, dvd drive, floppy drive, tv tuner card + software + wires
PC will reside in the space in the boot where the CD changer goes, linked straight to the battery. With ignition wire, on/off switch, usb extension lead (to usb hub), video cable going from the boot to the front dashboard where it will be linked to comand & keyboard etc.
PC could run windows XP with all the usual software installed (autoroute, media car, winamp, windvd, tv tuner software, tv out software (locked on out), office, games etc etc.
any further ideas please let me know. i plan to start shopping for all this stuff after xmas is over.