Ever have someone plug both ports of a phone into the wall and watch the switch light up like a Christmas tree doing the wave?
It's the network equivalent of holding a microphone to a speaker! Fun times.
Netgear v2 models have a feature to prevent these.
(v1s do not. v2s have the blue/purple interface, v1s have the orange one.)
Log into the switch, and go to Switching -> L2 Loop Protection. On Admin Mode, switch it to Enable and hit Apply.
That is *half* of the puzzle, enabling it globally.
Step 2 is selecting all* the ports, changing Keep Alive to Enable, and hitting Apply.
* = NOT PORTS CONNECTED TO ANOTHER SWITCH. If you do that you run a very good chance of locking yourself out of the switch till it restarts, because it will see the keepalive from the other switch, think it's a loop, and shut down.