Help! Network and internet problems

Hi, I need some help:

On our network, we have 4-5 computers, constantly 2 mac laptops, a windows 7 laptop and a windows XP PC and sometimes a windows XP laptop. My problem is, one of the macs is slowing the internet ALLOT, for example, server we (all the other computers) normally get 160-180 ping to, get 300 (at a good time) to (the most we've gotten) 3500.

Sometimes even, crashes the internet when they close the lid/put it to sleep. When not using ethernet cables, both the macs and windows computers get a network error along the lines of "Windows has detected an I.P address conflict" obviously, it wouldn't say windows for mac, but we have worked out that both the messages we get and the same thing.

We have changed routers once or twice since this has been happening, but it only used to crash the internet (you could still connect to the network, just no internet access), so it is not the router, but yet again the mac. When the mac is asleep, not here or not on, we have near to no probelms (within around a week, maybe one crash). The only other time where we get amazing lag, is when the other mac load youtube videos.

The mac causing the problems is only around 3 years old. We didn't have the problem since it's been on the network, but just within the last year or two. It only started out crashing the network once or twice a week, and has gotten up the point where 1-3 crashes a week and lag nearly whenever it's using the internet, even when its just facebook we get around 300-800 ping.

So if possible, please help.

Thanks
Harrison

EDIT: The mac causing the problems, gets no reduced internet speeds. Hence, I think the mac is somehow "Stealing the internet",

I wouldn't say this is a Mac problem, sounds more like a DNS problem. Why is it handing out IP numbers that are already in use?

Quick Fix, tell the DNS not to hand out a range of IPs, then give the "troublesome" Mac a fixed IP number pin that range. This should fix and bypass the problem. See if the problem persists.