ADC Sound Recorder is a free Windows program for recording sound from all sources available with a computer's sound card.

That includes a microphone, line-in, wave, etc.

Recordings are automatically saved to the program's cassette file that can keep up to ten minutes of recorded material in 11KHz, 8-bit, mono quality.

To free some space for new recordings and still keep the old ones, you can export them to standard WAV sound file format and then delete old recordings from the program.