You really should have some caps on your 7805, at least something on the output so it doesn't oscillate. You should always use a resistor between the Arduino pin and the transistor base. Do you need high-side switching? If not, consider something like an NPN darlington TIP-120 or (better yet) a logic level FET on the bottom side. The gain of the TIP42 is not very high, that's why I recommend anything else. You want to install clamping diodes across the injector leads to safely vent the high voltage spikes to the supply rail. Look around for examples of how it is done with relays.
EDIT: Does your Leo work at all? Start by disconnecting Pin 3 from the transistor. Can you reprogram it with the Blink Example? If so, then you may have damaged pin 3, otherwise, sadly, if the board acts completely dead and won't do anything, or parts on the board are getting roasting hot, then it's probably curtains. Still might be only the mega328 which can be replaced.
