You have a CPU error code, beep or light?
It's still hard to say, because without a power supply that you KNOW has adequate quality AND capacity, and is functional, with no problems, then you still can't rule it out. Every single component in the system can fail to work properly if it is not getting sufficient power. Assuming you believe the power supply to not be the problem, which I have a difficult time agreeing with completely given how often I see the PSU turn out to BE the problem, especially when it's either very old or very poor quality, then I believe your next step will be to disconnect from power completely, remove the CPU cooler, remove the CPU and check for bent pins on the CPU. If there are none, then it looks more like a motherboard issue but it's impossible to rule out the CPU and there is really no way to test for a faulty CPU or motherboard other than eliminating everything else.
Also, you don't want to pull the CPU until and unless you have or can get some thermal paste so you can replace it before reinstalling the CPU. You will want to clean the old paste off using isopropyl alcohol and a micro-fiber or lint free cloth, or paper coffee filter. You don't want to use normal rags or paper towels because they can leave lint behind on the CPU or heatsink surface which is enough to reduce the effectiveness of the cooler. Reapply using a dot in the center of the CPU about half the size of a frozen pea or a line about twice the width of a grain of rice and about 1.5x as long. If you are using a low mounting pressure type cooler that uses push pins or clips rather than actual screws to fasten down the cooler, then use a little more than you would for a standard cooler since it won't tend to flatten and push out the paste nearly as much as one with a high pressure mount.
Before doing any of that though, it might be worth double checking the basics such as removing and reinstalling the memory modules. If you have two memory modules installed then be sure they are installed in the second and fourth slots away from the CPU, if you have four DIMM slots.
Try reseating the graphics card.
Try disconnecting all drives to see if it will at least boot into the BIOS.
Double and triple check everything here:
"No POST", "system won't boot", and "no video output" troubleshooting checklist This checklist is a compilation of troubleshooting ideas from many forum members. It's very important to actually perform every step in the checklist if you want to effectively troubleshoot your problem. 1.Did you...
forums.tomshardware.com