For the past 2 years, I've had my CPU, a Pentium D (3 Ghz), and I have not encountered any heating problems. Until today.
In the middle of a game, Fallen Earth, my computer started to death-squeal, so I decided to clean out the dust from the CPU and GPU fans. Upon loading the computer back up, I checked BIOS and it claimed to be running at 75 degrees in BIOS. I decided to ignore it and start Windows. Upon entering the login screen, it started screaming again, even though this time I had set the warning to 80 degrees.
My suspicions are that my temperature monitor has malfunctioned, as when I shut the computer down, I feel the CPU and fan, and it feels relatively cool, considering that it was previously stated to be at a temperature of approximately 90 degrees. (Holding a thermometer on the CPU after shutdown resulted in a reading of 34 degrees).
I would make sure that the heatsink still is making good contact with the CPU. Best is to reinstall it after cleaning the old thermal paste and using a very thin layer of AS5. If the heatsink doesn't make good contact, then it will never get really hot.