analogue recording is the order of the day. You'll need a stereo lead from the output of the organ, line out on the back, or even headphone socket if nothing else is suitable, to the line in on the pc soundcard.
If the pc has an optical writer with Nero or Roxio installed these ususally come bundled with a Wave Editor program which can be used for recording. Just set the options for line in on the soundcard, wave, 16 bit, 44.1kHz, and the recording is ready for compact disk burning.
Otherwise download the free Audacity from http://audacity.sourceforge.net/
which can do as good a job, including mp3 conversion with an optional free codec.