Hang when unplugging ethernet cable

I have an inspiron 6400 and connect to my broadband provider through pppoe. If i unplug the cable from my laptop without first disconnecting the connection from the system tray, the laptop hangs. No blue screen, everything just stops responding, mouse/touchpad/keyboard don't not work. I need to shut it off forcefully with the power button. Anyone experienced this before? What's causing it?