My PC overheated ! 
HAS only 2  causes (not caused by building a bad custom PC.
The fans are dead or packed up.
The PC is packed full of lint.

The PC has fans that must spin for sure PSU and CPU fans and if you have a nice GPU card that is fast it too has  fan.
The fans must spin. (some newer PCs the fans run slow and then go faster as the PC gets too hot, this all 100% normal)
Most PCs run at 50C max and overheat at 90C,  and when overheated the magic Intel inside logic slows the CPU down, (called thermal throttling) GPU's now do the same ACT !
The #1 cause is NEGLECT , not keeping the PC clean.
Using shop air is best, outdoors, not blowing that filth into your home or use, CAN-O_AIR from Walmart.
Some PC's shipped in the mail, the CPU heat sink fan can fall  off the CPU, mostly AMD not Intel   CPU do that, as the Intel are screwed down hard.
We look first inside the case for lint packup;.
On some silly laptops 35 screws must be removed, but no my Dell only 6.
If you must break up any HS heatink to CPU /GPU bond , this processor now needs a fresh super thin coat of TIM, thermal interface material, (  I use what intels uses,  "SHIN ITSU ™" G751 )  < zero hype only facts.
Do not do this.  but only do this BB sized  TIM,   >>>>>>    (a super thin paste works best at  0.1mm thick or less thin is the best ! )

I think my  photos speak best, they even shout Neglect, no words needed.
See my PC overeheated slide how here for any slow or   PC DEAD!  A PC can overheat in 1 minute flat, or faster if it is a super fast PC you have. (gamers)

Most of the photos here are so captain obvious.
Vacuum it out first. (then use CAN-O-AIR  or 20psi max shop air to blow out the rest)   we use a line with a HVLP paint spray regulator to limit the force of the air pressure to safe levels. (100psi or more will damage parts inside)

Now examples show and tell:  (5years of neglect?)  Obvious ! Note this PC has CASE fan too and is packed.  4 fans here are failing !
No questions here, pure neglect.

More neglect.

See fan see RAD if other RAD or FAN is packed up, the PC will overheat. This PC is clean, I had to show one clean.

The Below is my Elitebook HP 8540W. See the vents and grills,  keep them clean? I use a paint brush to clean the slots. 15 seconds work on my part.

Again the FAN and  main RAD (hiding far left)  must be clean, and all of the inside lint free. Seen keyboard off here.

To get the RAD clean below it had to be removed. and then needs no fresh heat sink compound applied.  Clean the RAD(iator) can be hard. lacking tools and TIM (a.k.a CPU heat sink grease)

The cure for LINT: 
Can o Air. duster

Take it outdoors, and blow it clean  using shop air or CAN-O-AIR (duster) sold in all good stores.  (never exceed 20 PSI pressure)
(I do use a vacuum cleaner first,  on the worst cases, see far above , then SHOP AIR with the 20 PSI regulator inline...) NEVER BLAST using 150+PSI shop air, ever.
Then blow all heat sinks and fans clean.  (and annual event here) always do this outdoors, never inside...
 It will be a nasty cloud like PigPen(tm) cartoon character. (wear PPE,  gear while doing this, or risk eye damage) wear GOGGLES !
You want all filth to land OUTDOORS.

Intel proof of Processor CHIP DIE LEVEL protection: "Thermal Shut Down" ( it will slow down then HALT) IT SAVES ITS OWN BACON.

In FACT Intel CPU (and AMD) auto self shut down and HALT when too hot. (in seconds) Read page 36

There are pages like this, for every Intel CPU made, this one goes in to great depth, no more guessing.

Covered here too, on modern Intel,  CORE2™. (see all pages  on thermal protections)

Question of the day 1:
Why would Intel let you burn up any $300 to  $500 processor? answer: Intel (and AMD) do all that is  possible to protect  itself from permanent damage by you, or by bad luck or not keeping the lint out.

GPU chips now do this same thing, self throttle if too hot.

More examples. SEE that copper RAD (Radiator ,just like in your car doing the same thing) This RAD expells all heat to the outside of the PC where it belongs.
As you can see getting the lint out of here is not easy just using can or blow air.  (it is too packed up for that)  Some PCs have an access port here or a black tape here that peels off and you can clean the RAD.
The heat Radiator device.

end of filth.

Look Ma. no filth here.

Three cheers to the maker of great motherboards from 1995 to now.

Homes that are full of endless wall to wall rugs and dogs and cats will pack up a PC full of lint and hair fast, (I  have hardwood floors and takes 3 times longer for me to get too much lint (by my choice)
YDMV your dust may vary, good luck keeping fast PCs clean and cool.

Cure1: Can-o-Air as sold at walfart. Simply blow out the dust and lint. Easy as pie for sure dismanted PC or Desktop sides off. $5

This page does not apply to those using a laptop , setting on a beach in a cabana at 120F, nor any water cooled PCs of any kind, nor fanless NOTEPADs or tiny tablets.

version 14.  7-25-2017   (now less, I just show endless bad, and you learn with photos all very captain obvious)