Two computers on one wireless modem router

[#1]

Hey all,
I have a Belkin F5D8631-4 wireless modem router on which my computer is connected via an ethernet cable and my brothers is connected via wireless.
If my computer is on and his isnt, my connection is amazing, but as soon as he turns his on... AARRGH!
Internet pages load up slowly, messenger suffers from severe lag and World of Warcraft has huge latency and of which becomes unplayable.
I was wondering if there is a technical reason for this, as I had the exact same setup with my old provider but never had this problem.
Thanks,
Kyle =D

[#2]

What is your brother doing on in his computer? If he is running P2P software for instance that would explain it.
We often have 4 computers connected here 3 wireless and one ethernet and don't suffer from those types of issues.

[#3]

Nothing as far as im aware.
I checked earlier and he was playing on Football Manager, which im sure doesnt use the internet connection.

[#4]

It might be worth getting NetLimiter2 monitor for free from here http://www.netlimiter.com/ and putting it on your brothers machine for a bit if you can.
That should show you any network activity that is going on.
For instance if the BBC iPlayer software is installed that could be up and downloading with no visible application running

[#5]

He only uses it for his football games, general web browsing and iTunes.
I'l give it a go anyways but im 99.9% sure he doesnt use anything that saps the connection.

[#6]

It's possible your brother has a computer virus (or other malware) and it is that which is busy using the internet connection.