>     If I take the +5V and +12V (and GROUND, of course) from the PC power
> suply and use it to feed the PIC in-line programmer board (David Tait's
> SERPIC), can I cause any harm to the programmer, target board ou the PC?
>
>     This would save me a 12V power suply and make the things a bit more
> ellegant (only 3 wires from the programmer to the PIC).

12 volts is not enough, you really need a little bit more.
-Bob