It's the CPU, I'm almost 100% certain.
Why? By a huge coincidence I've just had my Athlon fail with exactly the same symptoms.
I was initially convinced it was the video card as the integral cooling fan turned out to have seized solid. Bought a new card, no difference!
Took it in to my local shop for a check, they thought it was a motherboard failure but tried a different CPU, which worked.
I paid for the new CPU, brought it all home, only for the same symptoms to re-occur within hours. Guess what? A SECOND bad AMD CPU!
Third CPU fitted today has cured the fault.
Hope this helps!