Car Alarm Won't Disarm!?

I have a 2004 X-Terra with an aftermarket alarm that was in the car when I bought it used. Lately, as soon as I open the door, the alarm has been going off even after I disarm it with the remote. Sometimes it just goes off when I hit the disarm on before I even open the door. The car chirps to indicate the alarm receives the signal from the remote, but it still won't disarm and just keeps going off. Obviously, the kill switch is activated when the alarm goes off, so I can't start the car either. Eventually after shutting/unlocking/locking again etc. it will finally shut off, but with no rhyme or reason. I don't know what brand it is since I didn't buy it. Any ideas on how to resolve this problem?