hi peter,
Not knowing over what range of resistance you want to measure, consider this as a concept.
Say, you have a 100uA constant current source, derived from your +5v rail.
Connect the 'unknown' resistor, say, a range of 40R thru 40K from the 0V rail to the constant current source,
ie; the 100uA flows thru the resistor to the 0v rail.
Connect the junction of the CCS and the 'resistor' to the input of the PIC's ADC. [10 bit resolution ?]
The numeric value the ADC outputs is proportional to the resistance value and linear over the range of 40R thru 40K, [ 0.004V thru 4.0V]
Before other members start groaning, I repeat this a Concept, not a design solution.