It all depends on the quality of the system. You might have better luck trying to just disable it. Some of them can get wired into a few different systems in the car (doors, engine, ignition, etc).
Do you want to install a different system?
To disable, you could try removing one wire at a time from the box and see what happens. There's also a trick with some alarms though that they'll let you get it started, but the car will only move about 20feet, then it'll stop dead. Good luck.