mc34063 isolated converter
Fernando,
Check out the LM2585 )**broken link removed**) from National. They have an app note to show you how to build exactly what you need (+5V in, +/-12V out) at a very low parts count. The hard part of designing an isolating DC-DC converter, at least for a non-power person, is designing the isolating transformer. National has arranged with Coilcraft to supply the isolating transformer described in the app note, making this a pretty turn-key solution.