The possible bad parts are, CPU, Power supply, RAM, Video card and motherboard.
If the CPU is bad, nothing will happen, Take out all the RAM, try booting, If you get any kind of beep it means that it is at least getting to RAM.
I would reccomend buying A really! cheap AGP or PCIE Video card (Whichever slot type you have) and a Cheap 256 MB memory stick so we can test them in the machine.
They are the easiest parts to test.
If you don't want to buy anything then you have to start RMAing parts, Unless someone else has any suggestions.