it will not boot but I can enter bios

I bought this laptop second hand as a dead unit, on examination I found that the I3 CPU had died, put a known good CPU in and turned on, Great, it started the POST, I get the Press f2 or f12 options, then the Acer Logo flashes up for a second or two then it hangs with the cursor in the top left of the screen, can anyone give help/advice?

PS:I can enter bios and see all devices, IE, Hard drive, CD rom and RAM, it can also see any USB devices I connect.

Things I have tried include removal of all USB devices, use of known good Ram, even removed the wireless module.