Sometimes there are unusual Internet activity in my Computer. Modem's lights are always blinking and When I open system monitor, I see there some unknown program is using my precious Internet with 64KB/S (I have 512kbps connection). Still I am in a firefox session with only one tab opened, and the page in already loaded and there is no indication of busy sign in the page (that rotating orange circle).

In that situation I unplug my modem and reconnect it again. After several times doing this, that unusual activity stops. It annoys me very much.

How can I find out the process which is using the Internet?. How much they are using? How can I kill it? A graphical solution will be better.