Frequency to voltage IC's are available. You may need to amplify the signal from the guitar to the F/V converter. For which you could probably use an op amp. Then run the output from the opamp to the F/V converter. The output from the F/V converter will need to be 0 - 5 volts for the uController so you would have to design for that.
Well, ideas are the point of forums. Please elaborate on processing small signals with the pic input, I am interested as well I am sure as is D.M.