I would have to check the data sheet to see which pin the ADC VREF is on. But if you did not tell the PIC different it will use VDD as a VREF. Show use you code and we can be exact.

Just for giggles check the voltage from the 7805 to see if it swings under a load. It probably is not 5 volt to begin with but if it is stable it might be close enough.