If you have to use the ADC0804 this may help. Has a tiny basic subroutine for an example. http://www.national.com/an/AN/AN-280.pdf

The 16F877 at rentrom.com has a built in ADC. This will work if you could switch micro-chips to one with an ADC.

If you can use an ADC0831 this should help. http://www.parallax.com/dl/docs/books/edu/BAADv1.3.pdf
The code from parallax is close to PBP.

The ADC0831 is serial and the ADC0804 is parallel.
The ADC0831 is very easy to use.