2000 nissan xterra ran fine last night I go out to start it today and the doors are locked the anti theft horn starts going off and wont start but all the dash lights come on like usual. was parked in the garage all night"

Best bet is a dealer as the keys must be associated with the car VIN.

Anti theft isn't working correctly.. try taking the ground cable off battery for a couple mins and reconnecting to try and reset anti theft..

try locking and unlocking doors?

Sounds like the anti-theft system has been activated. The horn would normally blow in conjunction with the flashing lights.

try disconnecting battery and when disconnected switch ignition on the reconnect with ignition on may reset system sounds like a cental lock problem try tailgate

You should have disconnected the battery and fully charged it before starting the car.
The low voltage may have damaged some of the electronic components in the alarm system.
Charge the battery fully, then reconnect it and see if that helps.

dealer only ,do what i do and leave a old tape player in car ,no cd and then i leave nothing in the car and dont lock it but i do have a steering wheel lock on the car for good measure .Never a problem then with junkies after their next fix

you need to look in your owner manual because its could be your Anti-Theft or call your local Nissan dealership and they be able to tell you what to do also give these websites a try www.alldatadiy.com and www.autozone.com wish you the best of luck Michigan Man.

I had a problem on my Nissan Xterra 2000. Symptons sound similar. In my case, it ran fine for a while and then suddenly the engine starts to stumble and lose output. At one point it did not even have enough HP to move. Irregular idling too. Thought it was water in my fuel. Bought water remover from the store and added to gas tank. Problem did not go away. Instead, "Service Engine Soon" light came on. Had to have it towed to the dealer as there was no way the engine could keep up with traffic. Dealer replaced the "Crank Position Sensor" today. Parts $33.00, Labor: $300 (about 3 hrs worth of work). Seems to work fine now but I will wait and see. Dealer also stated that Distributor bearing appears to be noisy and will need to be replaced sooner or later. I will wait for it to go before I fix that.

jimmy: it sounds like the drivers door unlock tamper switch or the other switch may have failed. One side can affect the other under certain circumstances. Your trunk lock also has a switch which arms and disarms the antitheft system. Try locking all the doors, make sure the hood and trunk lid are closed and then open the trunk. Leave it open open the drivers door and try to start the car.
If it's a no start. Disconnect the ground battery cable then put the ignition key in the run position in you ignition switch. Reconnect the battery and try to start the car. It should start. This usually will override the anti theft system. One issue with this will be, whether the antitheft system will remain disarmed.
Although I have never tried this before, I know that the system will not arm unless all the doors, trunk and the hood are closed.
I could suggest this to try if the car starts as I had told you pertaining to disconnecting the battery cable, I would take the switch and unbolt it from the passenger's side of the engine compartment just forward of the shock tower. Just let it sit somewhere so it doesn't get caught up in anything moving. If there is a light under the hood of your car, remove the bulb.
Let me know how you make out.