The two most likely causes are torrents/P2p, which has been mentioned, and someone deliberately or accidentally, (it does happen accidentally), leaching off your wireless conenction to do the same.
I may be teaching you to suck eggs, but P2P means just that. If you download things using it, then when someone else wants the same thing you will be one of the providers uploading one of the streams that other person is downloading.
What wireless security have you got - WEP is useless, though of course none is worse. You need WPA2 with a complex password. You can also check what is connected via your wireless link, through your router GUI.
My broadband basic info/help site - www.robertos.me.uk
My domains,website and mail hosting - Tsohost
. Internet connection - Plusnet Value Fibre
FTTC 80/20 trial.
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.