An implementation of Goertzel's algorithm was in an app note for TI's dsp processors for decoding DTMF tones. Whilst a DSP processor is somewhat different to an 8051, the program logic is still much the same.

Do a google on "Goertzel DTMF TI" and see what you get, else go to TI

If still no luck, come back here and I can dig up the reference.