CPU/Key Problem?

Out of the blue, my XJ will start and then die 2 seconds later. My model year has some sort of cpu chip in the key itself and it will do exactly this if you try to start it with a copied key that wasn't made at the dealership (I have one of these as well). Also, there is a light in the instrument panel that should light up when starting (key with a \ through it). The light's not coming on now. So, all signs point to the chip in the key not making the appropriate contact with the CPU, but I've no idea where to go from here other than having it towed to a shop. Any ideas?

Disconnecting the battery seems to have done the trick.