Your security system, most likely related to the ignition switch or key. Try another key if you have one, and/or clean the resistor pellet on your existing key. The problem could also be the contacts inside the switch that touch both sides of the pellet are gunked up. Get a pipe cleaner (Picture of Pipe Cleaners) from a craft store and spray some electrical contact cleaner on it. Then run it in and out of the ignition switch a few times to clean the contacts. You could use a Q-tip, but that could leave some fuzzies behind.