This circuit should do the trick. Make sure you use low tempco resistors, especially for the reference resistor R1. Make sure the PIC uses the 5V supply as the reference for the A/D.
The opamp needs a negative voltage, like -5V and a positive voltage of at least 8V, since the 741 is not a rail to rail opamp.
To adjust the circuit, with the PTC at -20C adjust the pot to get 0V at the output. Actually, if you ask me, I would adjust it to get about 50mV above GND. That way you will always be sure you measure -20C, otherwise it's possible to get a negative voltage at the PIC's input, which you cannot measure. The voltage at the pot wiper should be about 0.41V. The value of the pot should be about 10K.
With the PTC at 120C, adjust (if needed) the 11K resistor R4 to get a little below 5V at the ouput. The reason is the same, give yourself some room to measure a little above your limit. Nomally, the gain of the circuit should be 11.92, but I chose it only 11 for the reasons I showed above. If you want, insert a 1~2K pot in series with R4.
This circuit should be good to prove the concept, but if you are going to mass-produce something, then some way to auto-zero it should be implemented.