Whats wrong with my citroen xsara?its gone nuts?

Your computer controls the alternator it has a regulator in the computer you also may have some grounding problems if you have a digital voltmeter check for voltage drop from the alternator to the battery if it's anything more than 1 V that will be your problem you will probably want to take this to a professional and have them diagnose it as you can burn out a lot of things in your electrical system and cost you thousands of dollars if this does not get fixedcorrectly