Help with computer .

Help I have a vista Gateway fx 6800 - 11 desktop computer .This is my problem I did a full system health test all is good .Except my plug and play is not working .But my main problem is my computer keep on shuting off .I have to keep on rebooting it . The weard part the front light is still on . I have done a vires scan nothing .All is good . I have clean the dust out of the computer . This is driving me up the wall . Sometimes it just shut off when I am doing something .I really need your guys help
Thanks Chris Wilson