How can i limit the amount of bandwith World of Warcraft takes up?

The problem is, when I am on the computer using world of warcraft and my brother is on the PS3 playing CoD or any other online game, we both lag. Now when playing WoW i dont mind lagging so is there a way that i can limit the bandwith WoW takes up? Please keep in mind as I am a credit card impaired teenager I can’t buy any software, free soloutions please.