changed the harddrive and grafics card
So you are starting the process of elimination... but from the wrong end. From the wall socket, start with the PSU, then motherboard - these usually (well, modern) have surge protection to protect the other components (e.g. CPU) down the power line by sacrificing itself.
Maybe list you motherboard, PSU and whatnot to see if people here know how to tell what signals to look for.