I glanced down at my network rate monitor, and my computer is happily doing 100-500KB/s of traffic. Somewhere. But where? From which process? This is Windows XP. I've Googled around, and it's not at all obvious what the right tool is. Task Manager will tell you how much CPU your processes are using, but not how much network bandwidth. The closest you can get is "I/O Reads and I/O Writes" (which may or may not have anything to do with the network).

Thoughts?

That's a good one. I think I'd look at the output of a traffic sniffer. But that won't tell you what process is doing it. I can't think of anything more useful than TdiMon. I don't think it has a UI to show you bandwidth utilization, but it will at least show you which apps are doing network reads and writes. I guess I'd just quiesce everything I knew about and look to see which apps are still going.

X-NetStat should be what your looking for.

That's definitely the sort of program I wanted. What it's telling me, however, it not what I was hoping to see. Here's a usage trace (on my 100Mb wired Ethernet):


Yet, according to X-NetStat (and, consistent with what TDImon was saying), the "bytes in" and "bytes out" columns are all zeros. Does this mean that I'm receiving a stream of packets that my machine is reporting but just quietly ignoring?

Try clicking on the Net Stats button and see if any of these report anything.

If you run Firefox or IE does the bytes in / out change?

Click on View and enable the first two options.

Ethereal works pretty well on Windows these days. You can give it a shot to see if you can figure out what the data actually is.

Ethereal turned out to be the solution. The problem, sadly enough, was Photoshop CS, and its image browser feature, running on my laptop and browsing images on my desktop machine. This, by itself, was generating all of the traffic in question. According to Ethereal, it was basically pinging each of the files in a directory, over and over again.