engine managment light?

my dads got a m reg vauxhall cavalier 1.8...it recently had a new alternator fitted and now the engine managment light is on along with the airbag light..it also idles at higher revs than before and occasionally it revs really high and the engine has to be turned off to bring them down...does anyone know if it's due to the new alternator or is there another reason why this is happening now?

Paul Evans
January 2011