PortA is just fine... as long as you've remembered to set the Port to DIGITAL as by default it comes up Analogue.

ADCON1=$07

Go see Datasheet ADCON1 Register Section 11.2

The only other gotcha on PortA is RA4, but since you're not using it, then it doesn't apply. Actually RA4 is one that I prefer to use for one of the I2C lines because it needs a pull-up, and you've generally got to provide a pull-up for I2C, so the two go together nicely.

If you have the 16F877A, you've got to do the same to the Comparators as well as Microchip have sneaked those under the hood whilst you weren't looking...

CMCON=$07

16F877A Datasheet Section 12-1