I have a network of about 50 PC's connected through an HP Proliant switch, with Cisco routers. There are 4 workstations that exhibit some very strange behavior. When you power any one of the computers down, it brings the network to a grinding halt. There's approximately a 90% packet drop rate. This lasts until you either disconnect the computer from the network via yanking the cable or powering the computer back up. Any ideas?
I would look at/replace the network cables and cards from these machines. It's possible that when the power is off they're shorting the pins in an incorrect manner, causing a loop or noise on the ethernet.