have you ever cleaned the inside of your case? the fans are probably caked in dust and so the CPU and everything is heating up a lot and cant be cooled.
turn off the computer, remove the side panel, start cleaning.
Warnings>
1 Be careful when cleaning the PSU, this can hold a deadly charge, even when off and unplugged.
2, have some toothpicks or something at hand if you use a vacuum cleaner, place the tooth picks into the fans so they dont spin when cleaning them. letting the fans spin can damage them.
3, you may notice a loud buzzing when you clean out and then turn the computer on, this is due to the dust acting like a buffer absorbing the noise, the bearings in the fan that is buzzing have gone (bearings are little metal balls in the fan, one or more have worn and become smaller and so vibrating), the buzzing fans need to be replaced.