Hi,
Open up the datasheet and take a look at the ADC section - especially the ANCON register. It says:
The ANCONx registers are used to configure the operation of the I/O pin associated with each analog channel. Clearing an ANSELx bit configures the corresponding pin (ANx) to operate as a digital only I/O.
Cool device that, 12bit ADC with differential input, two internal references - hadn't noticed it before.

/Henrik.