Outgoing Flood

[Mindflare]

A relative of mine is using a dialup connection with some unusual effects going on. (The computer is nice, runs Windows XP.) When she connects to the internet, the computer starts sending massive amounts of data outward (it sent 21MB in a half hour, which is impressive for a dialup). I'm not sure what's sending data out or where it's all going, but it slows all internet usage to practically nothing. I can't find the source of all this outgoing data. I've done repeated spyware checks, which came back with no results. I've cleared her running processes down to the bare minimum. I'm wondering what could cause this and how it can be remedied, or even if this outgoing data can be monitored to see where it's being sent or what it is. Thanks in advance.