Had exactly this problem before, using a crappy quality cable. The run was barely 10 meters, but it was one of those 'flat' network cables. Plugged it into the computer, and the light on the hub flashed on and off about once every second. The WinXP connection status thing also kept coming and going about as often (although it couldn't keep up).

Fixed this temporarily by reducing the connection speed to 10mbps half duplex in the network card settings, and permanently by buying some decent quality network cable.

HTH,

A.