Weird/ impossible problem

well imy brother has his own computer i cant really say the parts because every single one had been switched out except a head drive, his monitor and keyboard

now the strange ting is it starts up but the scree, mouse and keyboard (us mouse and keyboard) will not turn on, no matter what i replace in it. now how can this be after changes out the ram and the cpu and mobo you would think that you would at least see it go to the startup dos.... but this is not the case... i thought it was low poeer at first sinc ehte power suppy then burnt out after rattleing.... and then we replaces everything... i dont think its the monitor keybord or mouse. and hard drive doesnt kick in untill after boot!

 

Onboard graphics card or PCI/PCI-E/AGP ? If it's an expansion card try connecting the monitor to the onboard port (if present) as it seems the monitor is not getting an input signal .

Without knowing what the CMOS/BIOS settings are it's difficult to tell if the USB mouse and keyboard should be on before Windows loads .