Stuck and don't know what to do next

I turned on my mid 2009 MacBook Pro which would not boot, was stuck on a grey screen, I left this overnight the next morning a folder with a question mark appeared, I replaced the hard drive thinking it had given up, the same happened with the new hard drive. Now when I switch on the computer the screen stays black and does not boot (can hear the chime when first switched on)

I've hooked up an external display and put the install DVD (10.5.7) in which boots but I can't switch the display into mirroring mode so can't see anything

So I'm now stuck as to what to try next, any suggestions would be greatly appreciated