Humongous input lag

[Almaro]

Until recently, I've had an awesome connection using vent, but now, my input lag is horrible: I can hear people perfectly and crystal clear, but if I try to talk, I get lag of four minutes or above. Commenting, private chats and public chat are impossible, and people will respond to things I said five minutes previous, and my input quality is reputedly bad.

This happens on all my servers, and trace routes show that it's just fine. Any idea what this may be? My soundcard is a Realtek AC97, and I'm using an Altec-Lansing headset (the model name escapes me), and I have a Linksys adapter and Linksys router with Comcast as my ISP.