Car Alarms: car doesnt start when battery replaced, valet mode, starter wire

Question
I recently sold my 1994 sentra to a friend... I had bought it used, and don't have the manual. She had to replace the battery and now the car doesn't start. She thinks it has an after market security system, I do recall a little led on the dashboard that I always wondered about. Is there a way to reset the security system so that the car will start? The only keys are the regular metal ones, no electronic ones.

Answer
Without knowing the exact make and model of the security system it would be very difficult to offer specific instructions.  In general an alarm can be overridden by turning the ignition on and pressing/flipping a small switch typically mounted in the driver's area of the vehicle interior.  This will put the system in valet mode, cause the LED to illuminate solid and bypass all security features of the alarm.  If this does not take care of the problem I would recommend removing the alarm all together.  Here is a generic 'how-to' on removing alarms:

Typically an alarm system would be installed on the driver's side interior of the vehicle.  This system would have been tapped into the ignition switch wiring running down from the key.  If you remove the dash panel(s) under the steering column you should be able to locate where the harness was added that goes to the alarm.  Disconnect each wire that was tapped in and insulate the factory wiring.  You will also see that at least one of the wires was cut, this is the vehicle starter wire that runs from the key to the starter.  This wire needs to be restored, I always recommend soldering all connections but crimp connectors should also work.  Make sure that you remove all connections to the alarm and that any wires that may be exposed are insulated and mounted in a way that they will not interfere with any moving parts and do not run the risk of shorting out.  Remember to take your time and follow each connection running from the brain of the system, leaving one wire connected that you are unsure of the function could cause problems in the future so be sure to remove every wire.