Wireless router/modem question...?

Here in my house we have 4 devices that connect with our wireless router. 2 computers, a phone, and an xbox360. I'm on a laptop in the back of the house, about 40 feet from the wireless router, which is right next to the modem. The modem is directly connected to our other desktop computer. When someone gets on the desktop computer and downloads something, anything, my internet connection from the wireless router goes to 0. I have a hard time understanding this because I had also just recently purchased a wireless router extender, so my computer says it has full signal, yet I can't do anything. I don't know much about how all of this stuff works, but I thought the extender would boost the signal and keep this from happening. Is there a way to stop my signal loss? It's really getting old... especially since it's my brother doing it and he usually does it when I'm having to do research for class. Any help is appreciated!