peugeot 206 alarm won't disarm, what to do?

alarm has suddenly started to go off when i open the car or put key in ignitition of my peugeot 206 (52 reg). Ended up having to drive home with alarm going off. eventually it stopped on way home but now goes off whenever I enter car/try to start it. Battery now flat but alarm still goes off if I try to disconnect battery.
Neighbours dislike me now!!