Sounds like you have a tamper system problem. Have you recently done any DIY work near any of the alarm sensors (doors or PIR's)?? If so that is the place to start looking. Check for any loose covers on the PIR detectors themselves, or damaged wiring near any of the sensors.
If there is nothing obvious, then you will need someone with electrical knowledge to trace the tamper circuit to find the fault and locate which sensor is causing the problem. This fault should be easy to cure, but if you are not a competant electrical engineer, then you may need to call out an alarm specialist to look at it.
Mike
September 2007