VoIP sound recording in YM

Hi,

I'm looking for a program that can record the sound of a VoIP conversation on my PC into a file (mp3,...).

The program should monitor the Yahoo Messenger and start recording when a call is made through its respective port, without the user of my computer knowing that the recording is going on.
(the audio version of a keylogger)

Please let me know if you know such a program.

Thanks.