Outgoing Flood

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.

The computer is possibly a zombie, meaning it was infected by a virus and some malicious user somewhere has control over it, and is using it to relay spam emails or as a porn server or something equally nice. You'll be wanting to format the hard drive, I'd think.

netstat -a to see all the network connections open, that might give you a hint on what the machine is acting as (in particular if it's sending mails left and right). Use a firewall (zonealarm for example) in paranoid mode for outgoing connections (ie, block all by default), then allow processes one by one to see which one is infected. And get a antivirus scanner.