Computer keeps shutting off !

Hello,

Im having a serious problem with my emachine computer . It keeps shutting off and it doesn't try to restart . It happens when windows microsoft tries to boot up . I tried using safe mode but it doesn't work now either. I have vista home premium. I remember using microsoft fix it, trying to speed up and clean my system . and I upgraded my avg antivirus before all this happened. I really need help getting my computer up and running again . thanks!