My 360 And My Laptop Hate Each Other

This is a technical problem that I'm having, FYI. 
 
Two months ago, I got a new modem as my old one died a horrible death. I installed it and all that jazz and everything was rosy. The two laptops, PS3, 360 and my iPod all worked nicely. Soon after, however, I noticed that something was a fair bit wrong. Whenever I turn on one of my laptops, my newer one of the two, it cuts out Xbox Live when I'm on my Xbox and boots me offline. I have pushed buttons, changed settings, broke some stuff and it still happens. Since I have my brother using this laptop a lot, it can get very annoying. More annoying since I have started playing MW2 again.
 
So GB community, I need your help. If anyone has suffered from a problem like this before, or just happens to know that golden anwser, it will be greatly appreciated.

I had a similar problem because I set the MAC address on my xbox to the same value as my computer. Can the laptop access the internet when xbox live cuts out ?
Also you said modem, did you mean router?

" I had a similar problem because I set the MAC address on my xbox to the same value as my computer. Can the laptop access the internet when xbox live cuts out ? Also you said modem, did you mean router? "

I had a similar problem with two computers. One could access my wireless network, but the other was blocked out. It sucked because my parents were visiting and I had to share mine.