Network Problem

Running a very large network of around 70 Computers + in one building. 1Gbps Backbone for servers and vital network services, users and non vital services are all on 100Mbps.

All computers are running Windows 2000 SP4 or Windows XP Pro SP1, having to run on Windows due to software requirements, Latest purchase of 20 new Hp machines all with pre-installed / configured Windows XP Pro SP1.

From all machines around the network up until a week ago, each machine would easily ping 1-10Milli Seconds to the main server. Over the last week there has been a substantial slow down over the network, computers are pinging no less than 16-25ms to the main server via lan. The company software (which runs databases off the main server) is running very slow due to network slow down.

The really weird thing is, it happened just after we replaced alot of old computers with new HP XP SP1 machines, so we presumed it was that, we have started to change machines back to Windows 2000, we now have 3 or 4 machines left with XP on, network is still slow.

Now we have done all of the following:
Installed Network Monitoring on all machines (all data sent / received is consistant and the same +/- 5% every day & all machines are basically the same. (for those doing heavy database work)

Turned the whole network off, and all devices, switching each device on individually, as soon as any XP machines were turned on network usage went mental again, (this was not one machines this was each individual XP machine would do the same)

We have tested all the hardware for faults / used different network cards, checked / scanned network for viruses / worms / rats e.t.c none found..

XP was set to auto update so we removed SP2 on all machines, network usage was lowered slightly, each machine now pinging a constant 16-18ms to the main server.

We checked the main server, and all servers / routers and switches, all of which are fine.. Firewall log has no errors & no successful attempts of any probing / access of the network without permission & is upto date.

I have started reverting XP back to Win2k, the network usage is slowing going down, however i only have 3 or 4 machines left, and it has not made much of a substantial difference.

I am hoping it is XP, however i am sceptical as to expecting the network usage to drop 6 or 7 ms across the network because of reverting 4 XP machines to 2000.. as the last 15 or 16 has only made a 8-10MS difference, so 4 machines is not likely to make a difference..

Anyone have any ideas?
I heard that in an attempt to cut down on the spread of network worms / viruses and malicious software windows has installed a new tcp/ip setting that basically will not allow more than a certain amount of packets to be sent at one time.. apparently its a new updated and you can change the settings of how many packets it will allow at once before it try's to slow the packet transmission down. I suspect that if this is a new update this is the problem however i cannot find anything on microsoft website..