As far as i,m aware there is no way to adjust the alarms sensitivity.
Try your closing your dash air vents, and remove anything hanging down from the rear view mirror etc.
Your age car do sometimes suffer from insects i.e. small spiders behind the cover of the movement sensors, try removing these covers and hoovering the sensors, failling that you could just put tape over them effectively removing them from the system.
I have heard of a mod/update for the face lift C class regarding this fault i.e. false tripping, apparently involving these sensors, but i have no more info than this, so this might be inaccurate.
Hope this is some help.