Carefully check that all sound/audio settings are correct for your setup in Control Panel ('Sounds and Audio Devices' in XP).
If no joy, try a system restore to a time before you installed the sw. If all's OK with your headphones after the restore, try reinstalling the sw - but ONLY if necessary. If you experience the same problem on sw reinstall, visit the Logitech forums click here & see if anyone can help you there. G