Computer surge... computer turns on but then fails?

Okey so a couple of days ago i posted a question asking about a Computer Surge... no one helped... so i decided to work on my comp. on my own... i replaced the Power Supply... the Computer TURNS ON... the CPU fan TURNS ON... but then after a couple of seconds it turns off... what is this due to..? i have a Hunch its the Hard Drive... need help ASAP (: