Well first of all you need to get that - 5V off the PIC before you burn it up. I'm surprised you haven't seen smoke yet. The voltage references need to stay between the range of 0 to Vcc. You are going to have to get yourself an op-amp and offset that -5V up to a zero volt potential and then divide your signal down. The resolution is what it is. You will get 10 bit resolution as the signal at the input pin varies between vref- and vref+ potential. If you want higher resolution or the ability to use your -5V to +5V signal directly then you will need a separate ADC. Funny... I don't see PIC16F676 as a device offered by Microchip.