Audio Playback Feedback Loop? (n-track)

I’m using N-Track to record some old audiotape lectures of mine onto CDs. I have a cable from the earphone jack on a Walkman to the microphone jack on a Sony Vaio laptop. When speakers are plugged in to the laptop’s speaker jack, I can use the “LIVE” feature of N-Track to listen to the tape as it records. If I plug in a Jensen Digital Audio headset instead of the speakers, a feedback loop appears to result; it is impossible to use this way.

Can anyone explain what I need to change in order to get the headset to work properly? My wife cannot work at her computer when I’m trying to record, due to the distractions, and I wish to monitor the recordings.

Thank you.