PC needs heat to start, and fans are rocketing

[Bissabia]

My old HP PC now starts only at the ambient temperature above + 26 C.
When it is switched on at a lower temperature, fans are spinning, the green lamp indicating power is lighted, as well as lamps in keyboard, mouse, modem. But the screen is black... And in about a minute all fans begin to spin at a terrible speed... Why!?
Moreover, the PC sometimes shuts down when something gets connected to a usb port (speakers, a keyboard).
Is it a problem with the power supply unit?
If so, can it be repaired or where could I get a compatible one?

[WAWood]

@Bissabia 

My guess would be, especially if this PC is over five years old, it that somehow it has a micro-crack in one or more of the circuit traces on the motherboard.  When warm, the board expands and the crack closes.  When not so warm, the board does not expand and the crack causes the board to fail.  This could also explain the USB problem, since that uses onboard circuitry to work properly.

You would probably NOT be able to replace the motherboard -- read the issues below:

Before you spend any money to attempt to replace the HP motherboard, you need to be aware of the issues involved with doing that.

First, HP does not sell laptop motherboards except for rare exceptions. If you use this link to go to the HP site and do NOT see your motherboard listed, that means HP does not sell it: http://support.hp.com/us-en/product/HP-Pavilion-dv6-6c00-Entertainment-Notebook-PC-series/5191856/mo...

That means any motherboard you buy will be USED, having been taken from another PC. Those often do not work, or have internal problems that the seller does not disclose. So, you need to buy it from a seller that will refund your money if the motherboard does not work properly -- many of them will NOT do this because the item is USED.

Also, the licensed Windows version that came preloaded on your PC is an HP OEM Version -- and that license is tied to the original motherboard, not to the PC owner. When you toss away that motherboard, you toss away the license -- and HP will not provide you a second license. Since the versions of Windows you can buy will not activate with the embedded HP OEM license, you will have to buy a Retail version of Windows, and a license to go with it. You will then have to replace the installed Windows version on your laptop with the version you bought and license it. We are not able to assist in any of that work.

The bottom line is the process is a lot more difficult than simply swapping one part for another.

[itsmyname]

@Bissabia  --- another great reply from @WAWood 

You stated that the computer is running Windows 7. If that is correct, then there is NO product-key embedded into the motherboard.

Note that Microsoft terminated all support for Windows 7 in January 2020.

Also, how old is your computer? If the disk-drive is the original, it is an "antique" (2011 vintage?). It is surprisingly good that it still is working, ten years later. So, if your computer was working, you should spend some money to replace it, before the disk-drive fails.

So, my recommendation is to remove the disk-drive from the computer, and purchase a brand-new "ready for Windows 11" computer.  Connect the disk-drive as a "secondary" disk-drive in the new computer, and copy your personal files onto the new computer. 

[Bissabia]

Thank you for the detailed account. This cause (micro crack) seems very plausible. (And the PC is really old, and it is not the main desktop PC in our family, though). Of course, I'm not going to replace the motherboard, and I'm going to buy a new computer.
But a theoretical problem still remains. Why fans spin so fast as if PC was going to fly? (I was even scared when it occurred first, and I was not able to find a similar PC behavior in Google). - If the fans are spinning so terribly, PC probably thinks that it is overheated and needs intensive cooling. But in fact, it on the contrary needs heat.
And it could be a practical question too. To make a choice toward HP or some other brand, it would be useful to understand logic of HP computers, and it seems to be paradoxical.

[Bissabia · Accepted solution]

The problem is solved! It was very simple. The motherboard is OK. And no new computer is needed.
One RAM slot was dirty. After I cleaned it with a solvent, everything works fine.
PC's shutting down at inserting connectors to USB ports might be explained by physical shaking of PC that accompanied these actions. But the behavior of fans is not understood.
(At the very beginning, to exclude memory problems I replaced all RAMs by RAMs from another computer, but this had no effect. And when I tried to start the PC without memory, it reacted properly by signalling the error by beeps. I did not go on with such experiments as I wrongly ascribed the problem to the power supply unit, but today I decided to try to remove RAMs by one and found that the problem was in a slot.)
By the way, this PC is probably made in summer 2011 as your site does not recognize its serial telling me that PC is older than 10 years, but it uses Pentium G850 with the start of production date - May 2011.