The real cause of the problem, and an easy sol. (Score 1)

The real reason that your downstream transfers are becoming sluggish while you are uploading stuff is that the buffers on your cable/dsl modem are being flooded. Remember that you have a 10/100 mbit connection between your NIC and modem. So when you are uploading, you are in fact sending data to your modem faster than it can send off. What you need to do is keep your XP box behind your Linux Filter/Router and compile support for different packet queueing algorithms into your kernel. I use the TBF algorith which stands for Token Bucket Filter. You can use this to limit the rate at which your NIC sends data to your cable modem...thus eliminating the problem of flooded buffers on your modem. That fixes things up real nice.