Edson,
      Have the second pole of all your buttons goto an 2 input or gate.
and the output of the or goto the pic power pin (you'll have to make
sure it handle you pic's current load or you'll have to put some sort
of driver behind it).  The other input goes to a pic i/o pin.

When a button is pressed it will turn your pic on via the or gate.
 Your pic then changes the line going to the or gate high, so if the
user releases the button before your pic is done it keeps power to
itself.  When it's done set the line low and it will lose power.

Come to think you could use a s/r flip flop (all buttons to s) it would
probably be more reliable and when you wanted to turn off the pic you
could set r high.  Just be sure to get a good low power s/r flipflop.

Aaron

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