Hi,
Very useful posts here. Some Hondas may have the factory fitted (optional on some models) alarm which links in to the remote door opening and operates that as well. The fob is a black rectangular item with two square black rubber keys, one raised which arms/locks the systems the other depressed which disarms/opens the system. Three screws on the back to get in and change the battery - alkaline 12v.

Programming:
Adding handsets to the system
Disarm the system and immediately turn the ignition ON-OFF-ON-OFF-ON and within 10 secs press the raised button of an existing handset 3 times. The system will respond by:
Illuminating the vehicles indicators
Illuminating the system LED (on dash normally)
The system is now in the 'key learn mode'

Press the raised button of the new handset once and the system LED will extinguish.

Press the raised button of the new handset once again and the vehicle indicators will extinguish.

This signifies completion of handset learning.

NB: if no handsets are programmed onto the alarm system, it will revert back to 'normal mode' after 15 secs.

If you wish to add more handsets, simply repeat the above process. There are five memory places in the alarm system for handsets, therefore there can be only a max of five handsets working on the system at any one time.

Deleting handsets from the system
Loss or theft of a handset may require deletion of the handset from the system. To do this use an existing valid handset and learn it onto the system five times to ensure the lost handset will be overwritten in the system memory. The system operates in round robbin fashion. After doing this you must then relearn any other handsets you have onto the system.

[extracted from the Honda HA3 alarm manual - 2000]