Computer refuses to start, CPU to be...

Hello, if you are wondering what the full title is here it is: computer refuses to start, CPU to be problem source. (without period) also, this is happening on my Dad's old computer, running Windows XP.

Yet again, this is happening on my dad's computer. It all started back in 2008 (I think) when my dad turned his computer on, and when he got past the boot screen, POOF! A blue screen of death suddenly appeared. My dad tried diagnostics (happening at the time this was written), but none of them worked. As it says in the title, the CPU might be the source, but if diagnostics work on running, the CPU can probably not support the operating system itself.

Can anyone please help?