NEED HELP NOW!

[reviv3]

Alright I need immediate help with this because its for my friend that is only over for today. Anyway I have a router and a cable modem. We have a 4 computers in our house that work perfectly on the LAN however, today my friend broguht his computer over and when we disconnect one of our computer and connect his it makes all of our computers frequently disconnect from the internet every few seconds. We are unable to get on chat clients for more then a few second or even in game without getting disconnected. When I disconnect him the network is fine. Its not the wiring because ive tried 3 types and a previous one that had worked. What could it be we desperately need help! Please respond thanks!