Ambrogio,

Use a switch mode regulator. I've had great success with the LM2576.

http://www.national.com/mpf/LM/LM2576.html

You'll need 2 x low ESR caps, 1 x 47uH..100uH choke and a fast recovery diode. You can even build your supply on Veroboard if you need to.

Way less painful than trying to use a PIC.

Phil.