About a year ago, my computer started acting funny. It would freeze up, applications would randomly crash, and I even got a BSOD (Blue Screen of Death) or two. After a few hours of suffering through this erratic behavior, I realized it might be due to my CPU overheating. That's when I quickly downloaded and installed Core Temp (free).
Upon running Core Temp, my suspicions were instantly confirmed: My processor was running at a sizzling 80-85° Celsius (158° Fahrenheit)--still below its maximum rating, but only just. It took less than five minutes from the moment I decided I need to know my processor's temperature until I had Core Temp set up and running.
I have a dual core CPU, and Core Temp showed me the temperature of each core separately. Surprisingly, there can be difference of one or two degrees between cores, even though they're both on the same chip. It's hard to imagine this being significant, but it's nice to know you can track each core separately.
Just above the temperature listing, Core Temp lists a value labeled "Tj. Max," with no explanation. I found this part confusing at first, but once I Googled it, I discovered it stands for "junction temperature max," which is electronics-speak for the highest temperature the CPU is rated for (90° C / 194° F in my case). You can use this value to see at a glance how hot the processor really is, relative to its maximum. But just because your processor is 10 degrees under the maximum doesn't mean you're safe. The Core Temp FAQ states that the temperature should be kept around 20° C/68° F or lower below its Tj. Max. value while under full load.
You can leave Core Temp running in the background and go about your business using the computer, maybe doing something CPU intensive. Once you switch back to the application, you would be able to see the highest temperature your processor reached in the meantime. Although temperatures display in Celsius at default settings, you can easily configure Core Temp to display the temperature using Fahrenheit.
If you're running Windows Vista or 7, one thing you should note is that Core Temp requires administrative privileges to run. The Core Temp FAQ (question 11) explains that this is because Core Temp requires direct access to the hardware for reading the temperature and related information.
Speaking of Windows 7, CoreTemp can cleverly use the Windows 7 taskbar to show the processor temperature, frequency or load. It does this by turning its icon into a progress bar, filling it with color from left to right. For example, if the icon is half-full, that means you're at 50% CPU utilization.
After replacing the heat sink and CPU cooler for my processor, I got it from around 70-80C down to 30-45C, and the computer started behaving normally again. CoreTemp helped me save my computer and avoid an unnecessary and costly upgrade to a whole new system.