Has the car been making a short series of 'beeps' at you lately? If so it's probably the power sounder battery failing, the most common alarm problem.
If not, check the alarm switch under the bonnet, it may need adjusted, or just bypass the switch.
Pushing the button with the 'running man' symbol as you leave the car will switch off the internal movement sensors which may help find the problem.
The quickest and easiest thing would be to take it to a garage with the appropriate diagnostic equipment to read the alarm activation log.
bbc333, June 2009
