Trouble with my car alarm?

Ive got a 200 ford escort estate that i have just bought and it has an aftermarket alarm. it is got sensors inside the car and switchs on the boot and bonnet. when i set the alarm with the key fob there is a light on the window which used to flash to let me know the alarm is set. whenever i set the alarm now the light stays on and 10 minutes later the alarm goes off. any ideas as i cannot set my alarm as it keeps sounding off and upsetting my neightbours
Answer
I had the same problem so i just dont set it now.