2000 Elantra: The problem is that when i press the unlock button...

Remote Control Programming (1997 ON)

Procedure
1. Switch Ignition ON and leave the key in the ignition.

2. Mechanically lock the vehicle using the Drivers door using a second
key.

3. Using the remote control press the Unlock button once, and the horn
will sound once.

4. Wait 6 seconds and press the unlock button once to activate the coding
procedure. The remote control will now be programmed.

NOTE : When performing programming on additional remotes the procedure
is the same, except you press the unlock button 2, 3 or 4 times for the
different remote.

For example : If programming 2 remotes press the unlock button twice with
a 1 second interval between each press, repeat for 3 and 4 remotes.

5. Switch Ignition OFF.

6. Remove key from ignition and check for operation.

7. The coding procedure can be repeated up to 4 times for additional remotes.

8. The ignition must be switched OFF between programming each remote
control.

if it's a 2001 my info doesn't show the 2001 Elantra GTS as coming with an alarm. GT does have keyless/alarm. Are you sure its not just a keyless entry system? Test it by doing the following: Close all doors and sit in car, 'arm' the alarm with the remote, wait about a minute and open the door from the inside...if the horn starts beeping you have an alarm. If nothing happens, you only have keyless entry.
Anyway disarming a ETACS alarm system is not a option.

nope. if she locked it with the remote it armed an ignition bypass. it has to be unlocked with the remote to disarm the bypass. a new battery for the remote is about 7-12 bucks.

Obtain all keys/remote controls. Remove driver's side kick panel/lower 'A' pillar trim. Connect central locking registration connectors (white) - black wire and black/blue wire Fig. 2 . Unlock the vehicle using the key. Program first key/remote control as follows: Press unlock button Fig. 1 [1], then lock button [2]. The door lock should lock and unlock in sequence confirming programming of first key/remote control. Repeat programming procedure for remaining key(s)/remote control(s). Once the last key/remote control is programmed disconnect central locking registration connectors. Check operation of all programmed keys/remote controls.

most of them you point at car hold down lock and unlock at the same time.

1. Insert a key in the ignition and turn from off to on eight times in rapid succession (within 10 seconds) with the eighth turn ending in the on position.
2. The doors will lock/unlock to confirm that programming mode has been entered.
3. Within 20 seconds, program a remote transmitter by pressing any button on a transmitter. The doors will lock/unlock to confirm that the remote transmitter has been programmed. (If more than 20 seconds pass before pressing a remote transmitter button, the programming mode will exit and the procedure will have to be repeated.)
4. Repeat the previous step to program additional remote transmitters. The doors will lock/unlock to confirm that each remote transmitter has been programmed.
5. When you have completed programming the remote transmitters, turn the ignition to off or wait 20 seconds. Again the doors will lock/unlock to confirm programming has been completed.

PROGRAMMING
Method One
Sit in driver's seat
Close all doors
Press and HOLD UNLOCK button on door lock switch.
Insert and remove ignition key twice. Do NOT turn ignition switch
Insert ignition key a third time and leave it in the ignition switch. Do NOT turn ignition switch
Release door lock switch.
A chime will sound three times indicating remote programming active
Press and HOLD the LOCK and UNLOCK buttons on each remote for 5 seconds
A chime will sound twice for each remote: indicating successful programming
Remove the key from the ignition switch to exit programming
Method Two
Remotes can be programmed using diagnostic equipment.

ok here is the procedure

make sure your remote works and the battery is good.

turn key on, press and hold the unlock button for about 5 seconds while still holding the unlock button press the panic button one time, release both buttons.

a single chime will sound indicating programming is ready, now press any button on all transmitters, after you have pressed a button on all transmitters to be programmed turn the key off.

programming is now complete.

This alarm system can only be 'disarmed' using the keyless remote programmed for this specific vehicle. Press the "Unlock" button on the keyless remote -- this resets/disarms the alarm system. If you do not have keyless remote for this vehicle, get one and have it programmed at a Hyundai Service Center. Problem solved.

It wouldn't be the car battery, but since you just purchased the car, the dealership should correct the problem. Bring it to their attention right away. Did you get any kind of warranty? Most dealers have at least a 30 day warranty for things like this.