Bellsouth and BEFW11S4

This is my setup 2 wired computers, 1 wireless laptop, wired xbox 360 and a wireless Wii.
my router is BEFW11S4, my modem is westell, DSL Bellsouth.


My problem is when i get on a computer and start surfing the internet everything is fine, but if somone get on the other computer i loss connection. Also if i start my computer up, sometimes the internet does not work at all, so i have to go and restart the modem and router and then everything works. Same thing with the 360. If i'm playing a multiplayer game, and somone gets on the computer, i lose all connection on the xbox and have to make the person turn off the computer.

I'm a noob to networking, But if anyone has any ideas it would be great. Thanks