A few days ago, I had to replace the thermal paste in the CPU heat-sink assembly of my Windows PC. Thermal paste (also called thermal compound or thermal grease) is a layer of paste like material that fills the air-gaps on the surface of both the heat-sink and the CPU, and joins them thermally. This way most of the heat generated by the CPU can be dissipated by the heat-sink. Experts suggest that we should replace the thermal paste for the CPU heat-sink every couple of years. After replacing the old dried up thermal compound with new one, you should also monitor the temperature of the CPU for a day or two.
If you have not applied the thermal paste correctly for some reason, the CPU temperature keeps rising steadily until the heat protection mechanism in the CPU shuts down the whole system. You can use the desktop gadget that comes with the Open Hardware Monitor to easily keep a close watch on the temperature of the CPU after applying new thermal paste.
Open Hardware Monitor is a simple program that displays the data collected from various sensors on your motherboard, hard disk, GPU etc. It can display the temperature of the CPU, clock speeds of various cores inside the CPU, hard disk temperature, GPU temperature, motherboard temperature and so on. What it can display varies from one motherboard to another.
In order to make it display the CPU temperature in form of a desktop gadget, you can right-click on the CPU temperature and choose Show in Gadget from the context-menu. Then select View → Show Gadget from the menubar. Pretty soon a small-size gadget appears on your Windows desktop that displays the current CPU temperature as it increases or decreases. You can also display other items like the hard disk temperature in this desktop gadget.
The normal value for a CPU temperature varies from one CPU model to another. For example, octa-core CPU’s generate much more heat than a dual-core CPU. It also depends on the room temperature of the place the computer is being used in. But in any case, if the temperature is not going down and only keeps increasing then you know that you have made a mistake in applying the thermal paste. You can also google the normal temperature range for your CPU which should give you some idea about what to expect.
You can download the Open Hardware Monitor program from http://openhardwaremonitor.org/.