>I am looking for a way to use the pic to control a resistor in a circuit
>(to use as a microcontroller controlled variable power supply).  Would
>anyone have any ideas how?  Thanks in advance.
>
You could pulse-width modulate an RC integrator to develop the reference
voltage for a regulator, use a CMOS analog switch pulse modulated with an
integrateing capacitor to form a variable voltage divider, an R-2R network
to source current to a current-mirror controlled regulator or pulse modulate
a MOSFet and filter its output.