I've got an HP desktop that decided to stop working. First, it started "freezing up" (no response to the keyboard/mouse and the video never changed). The PC could be rebooted, but it didn't take long for this behavior to start again. I ran a quick disk scan for viruses/malware and didn't find anything. Then the PC began shutting down (power supply shut off) by itself. Now the PC won't start. When the power button is pressed, the PC's power supply cycles on and off by itself every 3 seconds (3 seconds on, 3 seconds off, 3 seconds on,. For ever)

I've pulled the hard drive and connected it to another laptop. It looks like the contents are fine. When I disconnect the power supply from everything except the motherboard, the power supply still behaves the same (cycles on/off in 3 second intervals). When I disconnect the (4 wire) CPU power cable running to the motherboard, the power supply starts up fine. Similarly, when I leave that (4 wire) cable plugged in and pop the CPU off the motherboard, the power supply starts up fine.

The PC's not that old (but out of warranty). I'm willing to replace (probably upgrade) whichever components are bad, but I'm not sure whether to blame the power supply, the CPU, or the motherboard. Any ideas?

I think the problem is on the power supply. Verify that by seeing if there are any bulging capacitors, or poor soldering contacts.
You can also learn more from this power supply repair guide in PDF format.

I purchased a new power supply from a brick and mortar retailer (so I could take it back if it didn't work). The new power supply does the same thing, so I'm guessing it's either the MOBO or the CPU. I just ordered both so hopefully that'll fix the problem and give me a little upgrade too.

Before you go any further. Do you have another or separate video card to install on it? It sounds like a video card issue. I would try that first.

Yes, my PC was configured with a separate video card. The old MOBO had integrated graphics too. I yanked the video card and tried to start the PC, but it behaved the same way.

I've installed a new GIGABYTE GA-A75M-UD2H MOBO and AMD 3850 APU. The MOBO is compatible with my old memory but gives me the option to upgrade. The new MOBO also gives me access to 6Gb/s SATA and USB 3.0 ports, which I didn't have before. The new MOBO and APU start up fine. So the fix/upgrade ran me about $230. Given the ratings the 3850 is getting, I think I came out ahead in the replace vs repair decision.

Of course Windows didn't like the new hardware - I'm guessing the original OEM license was tied to the old MOBO. I've installed a trial version of Windows 7 which seems to run fine. I've ordered another OEM license of Windows that I'm waiting for now. That's an unfortunate $99 tax that I need to add to my repair bill.

Not an answer. Just wanted to include;

When Windows is installed on a computer, it assigns hash values, to certain major hardware components.

1) Processor

2) Motherboard. Specifically the motherboard chipset, and the BIOS program, and version.

3) Graphics chipset

4) Hard drive
, to name a few.

Change any two of these components, and Windows 'thinks' it's being stolen. Throws up a red flag.

{ Microsoft license states, "One GENUINE copy of Windows per ONE computer".

Windows 'thinks' you are trying to install this genuine copy on another computer, besides the one it has already been installed on.

Microsoft makes ONE Master disk. From it all GENUINE copies are made, and sold to the consumer }

Changing a motherboard, may be changing a bunch of hardware components.

A) Could be a different motherboard chipset
B) Different BIOS program, and version
C) Different graphics chipset. (Referring to Integrated Graphics. Graphics chipset is soldered to the motherboard)
D) Different Processor.
,and so on.

4) If you are lucky the Windows Activate screen will come up.
(Activate Now)

I click on - Call Microsoft. I like speaking to a real person, and not a computer voice. Also if something wrong happens, it can be corrected on the spot.

After you click on Call Microsoft, you are given a toll-free phone number to call.
The screen in front of you also has this information;

There are 6 blocks with number/letters in them.
These are to be read off to the nice representative on the phone. CLEARLY. One block at a time.

Read off one block at a time. Make SURE the representative understands you, for EVERY block.
When finished the representative will state please wait. (Or HOLD ON, lol!)

The representative will read a series of number/letters back to you.
Type them in the first block.

Make SURE you understand what they are. IF not, ask the representative to kindly repeat them.

After you fill in the first block, the cursor automatically goes to the second block. Type in the letter/numbers.

When all 6 blocks are filled, there is a moment of silence. You, as a 'tech', are supposed to know to go to the bottom right, and click on Activate.

Congratulations! You're done! By the way, the old product key affixed to the outside of your computer, will No longer work.

Regards,
joecoolvette

Yeah, I tried to boot up the PC with the old copy of Windows. Windows realized something had changed and tried to "repair" itself. After churning for 10-15 minutes, Windows came back and said that it couldn't be repaired. I don't recall the exact message it gave me, but it roughly translated like "tough luck".