FS 2002 Altitude Problem

Hi allI have got a weird problem with FS2002 again,...when I enter the Altitude in any plane I can increase the height but I can't decrease it.say I have entered 6000ft, I can increase it but not decrease it back to zero,this happens on every aircraft I have installed in FS2002there must be a corrupt file somewhere, or do I have to reinstall FS 2002 and all my other bits again?.any helpsee yaGrahamanyone can make a mistake said the dalek climbing off the dustbin.