I am trying to program a new key and instructions say the security light should come on but it is not. The vehicle is a 2000 Pontiac Montana.

Does the security light come on when you turn the key to run ,Does it go off in a few min. The proper way to reset the anti-theft system would be to turn the key to the on position for 10 min. if the light goes out in that time ,turn the key off for a few seconds . Then try an start the vehicle . If light comes back on an won't start there is a hard fault . You mite at that point wan't to do a theft system by- pass . Also have your battery checked and or charging system . A low battery , a malfuctioning charging system may cause these type problems .

The security, antitheft or lock picture/icon is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. This link explains it pretty well.
http://www.diy-gm-security.com/

I have a 2004 Pontiac Grand Am and I have had the same issue as you. I discovered that some pontiacs from 2000 and up have a security feature that, if triggered, will not allow the vehicle to start. In order to fix this propblem the computer has to reset. This can be done by placing your key in the ignition and turning it to the ACC mode. Do not place in the ON mode. Once the key is in the ACC mode the dashlights should all be on. Let the vehicle sit in this ACC mode for 15-30 minutes. The computer should then reset itself and the vehicle should start.

did you re program new key? blank keys have chips inside. or some blank ones can have only space for original or copy immobilizer chips. so you need to transfer your original immobilizer chip.

As long as you still have one that works, it's easy.
1) Insert the "original" (working) Key.2) Turn the Ignition to ON (RUN), but do NOT start the engine. Turn the Ignition OFF and remove the key.3) Within 15 seconds insert the new key and turn the Ignition to the ON (just to ON, not to START) position. The Security indicator should light up for about 2 seconds to indicate that the new key has been programmed.
At that point, it should work. I don't know how many different keys you can get the Theft Deterrent Module to recognize; there may be a limit.
If you're don't have a working key now, it's much more time-consuming;the procedure to get the TDM to recognize a different key takes about 30 minutes when you don't have an old one that works.

Turn on the key for 10 minutes then try starting

Very common problem on these cars. Do you have another key you can use? Try it. If not take the key ou have been using and turn ket to the on posistion. Leave it there on for ten to fifteen minutes. The computer will relearn the chip code on key. When light stops blinking you know it is done. Turn key to off wait ten to fifteen seconds and turn key to on position, Security light may blink once or twice then it should stop.. Try to start vehicle. It should start. If it doesn't then you may need whats called a new passlock and possibly key reprogramming. Which can only be done at the dealer most of the time. Hope car starts with the first procedure. Let me know if it works.

dunno if this will help or not but dis connect the battery re connect and if your key has a chip in it the chip may be bad so if you have a spare key with a chip try that one

For instructions on how to program remotes and keys check website below, I use these and they are usually very accurate. This website does state that on your vehicle, special equipment is needed to program a key fob, as there is no onboard programming for it. However, there are programming instructions for transponder keys.
http://www.programautokeys.com/

http://www.programautokeys.com/

Ignition keys relearn takes about 30 min to do. 30-Minute Learn Procedure 1. Turn ON the ignition, with the engine OFF. 2. Attempt to start the engine, then release the key to ON; the vehicle will not start. 3. Observe the SECURITY tell tale light. After approximately 10 minutes, the light will turn OFF. 4. Turn OFF the ignition switch, and wait 5 seconds. 5. Repeat steps 1 through 4 - two more times - for a total of 3 cycles of 10 minutes. 3x10 = 30 minutes. The vehicle is now ready to relearn the Passlock Sensor Data Code and/or passwords on the next ignition switch turn from OFF to CRANK Important: Turn the Ignition Switch OFF. The vehicle learns the Passlock Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle. 6. Start the engine. The vehicle has now learned the Passlock Sensor Data Code and/or password. 7. With a scan tool, clear any DTCs if needed. History DTCs will self clear after 100 ignition cycles. Without a scan tool you may have momentary starter interruptions while history clears itself.