Computer bringing down the network

I currently have 6 PC's with Gigabyte GA-MA69VM-S2 motherboards (Realtek 8110SC Chip LAN Port) and after loading WIN2K we attached it to the network to attempt to connect to the internet to load updates, this is where the problems began.

Once the PC was plugged into the network the Default Gateway (Cisco 1000 series Router) stopped responding. I could ping any other PC on the network, but no go on the gateway, and needless to say all internet connectivity was lost. After unplugging the PC and waiting approx. 2 mins the gateway would respond again.

Thinking it might just be that PC we tried another with the same components and had the same result.

I verified there was no shared IP or MAC address and used newer drivers.

I have googled the symptoms and have had no resolve. If anyone has any insight as to what might be causing this it would be greatly appriciated.