One thing to keep in mind with the revision I described is it is incredibly sensitive to noise if the signal goes away. It will chatter at some high frequency, so you either need to code against that or somehow just know when no signal is there so you ignore the input.
Or... you could DC couple the sine signal to the comparator but use the comparator as an amplifier. Feed the amp output direct to a digital input. That should chop the sine wave into digital form while also driving any noise problems away (you either get a square wave or nothing out of it). Input R of 1K, input C of .01uf and a 10K feedback R should get you there with a gain of 10. Increase the feedback if your output signal isn't enough. (Ask me if you need this drawn up.)
A quick check of the 16F628a data sheet shows RA2 is also AN2, so make sure you shut off the comparator function (the default) so it works as a digital input.
I've used schemes such as this in a few products to measure a frequency. What I did there was use the signal to increment one of the counters while another timer would be set up a roll-over interrupt off the internal PIC clock. To start, I would clear both counters and enable the interrupt. When the interrupt occurs read how many counts I got in the first counter and reset it. That way every interrupt time I get a frequency measurement of cycles (counts) per unit time.
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