It seems this might be a Sonar / Cakewalk or Windows XP bug. The Tuner gets its input routing from the Windows Wave Mapper (Windows Control Panel - Sounds and Audio Devices - Audio - Preferred Recording Device) and sometimes when an input is selected both there and in Sonar, a conflict arises. Try stopping the Sonar audio engine by clicking the o)) button next to the ! button and try the Tuner again. Or select an unused audio input in the Wave mapper.