Choose the "Sound" item in Windows Control Panel to make sure that no hardware or software effects (such as echo cancellation, karaoke, equalizer, etc.) are enabled for the recording device.
Check all cables to ensure they are firmly connected. For stereo recordings, make sure all cables and connectors are stereo.
Use "PCM 16 bit" or "PCM 24 bit" recording quality in GoldWave (press F11, Device tab). Those settings take exclusive control of the recording device, stopping other programs from altering the recorded audio.