That could be just about anything. What have you scanned with as far a virus/spyware/adware/etc. so far? Have you brought up the Task Manager to see what it is that is running?
Using XP Pro SP2. I have noticed that when offline, and hiding/showing folders or files my computer trys to connect to the internet. When ON the internet the activity modem tray lights become very active.
I of course KILL the connection at that point.
Spyware ? hacking ? seems like but what sort of spyware ?