Why does my alarm keep sounding when car is dead locked?

I have recentley purchased a 2000 1.4 peugeot glx. I love the car but when the car is dead locked the alarm keeps activating after about 5 minutes. It's so annoying I no longer dead lock the car which I'm not happy with but what else can I do. Does anyone know if the alarm can be adjusted. I have no book on the car.
Many thanks