P2P apps like eDonkey can cause this kind of activity. After you close the software you will still get other PCs attempting to connect for a couple of days because those other clients cache your IP for a while. This threw me off once because I could see my cable modem going crazy, even though nothing was running. Your firewall log should tell you what port the traffic is going to.
Core2 Q9400 @ 3.0, eVGA GTX 260, 4G DDR 800, Vista64