I've done this in C in DOS before. Pulling tones out of shortwave noise isn't easy. I eventually used an integer FFT to isolate certain frequencies from the static. That worked pretty well.

If you just need to know when a local key is down, you might have an easier time using a signal line from a spare serial port, or one of the data lines on a parallel port. This would eliminate the DSP issues. All you'd need is the right connector and some wire. The ports even provide their own voltage.