I'm gonna say your fan doesn't work...it doesn't look like you have a build up of dust and a broke fan or build up of dust are the only two things I know that cause a PC to overheat...
In case you haven't figured it out, I know next to nothing about the inner workings of computers, please ignore my opinions
Tsk tsk tsk... Allow me to teach you another lesson in the inner workings of a PC.
That silver chip with the gray dot on it... that's the processor. Modern processors generate a lot of heat, and this need a sophisticated heatsink/fan combo in order to dissipate the heat. A heatsink/fan that's sort of like what's attached to the back of the case right now.
The strange thing is that the gray dot on the silver chip is heatsink compound... you put that between the chip and the heatsink to fill in the microscopic scratches, ridges, etc. so that there are no little pockets of air that would make the heat transfer less efficient. That implies that the heatsink was installed at one point.
This lesson was brought to you by the letters P and U and the numbers 6 and 9.