depending the PIC you're using it can be ADCON, ADCON0, ADCON1 refer to the datasheet.

How can it make difference... roughly, you're checking a digital signal, if your analog converter are not turned off, you can have some odd/erratic results. a close to be low level signal can be considered as a high level and so on for the close to be high or high level. digital is digital, analog is analog. It's a common problem like some others, let's say RA4 to drive a source on many PIC