Get yourself one of those converters that output normal ATX voltages out of 12 VDC. They are just a board with an ATX connector, usually used with ITX mobos. That will save you lots of troubles, and they cost around 15 Euro, IIRC.
If you DIY, get some TL494 to convert from 12 to 5, some transistors, a LM555 to ger -12 from +12 (as oscillator, later a diode and a couple of elkos).
Don't trust the stability of those 12 V from the battery... use another 494 from 12-> real 12V ;-)
There are some canned DC-DC converters from National and Texas... really nice stuff, they even give samles for free !
