Hardware Sensors Monitor (also known as Hmonitor) is a Windows program that allows you to monitor the temperature of your CPU and motherboard, as well as the speed of your system's fans and the voltage levels in your computer's power supply.

The program requires a motherboard that reports temperatures and fan speed RPMs. A list of compatible motherboards is available on the hmonitor web site.

Hmonitor creates an icon in your system tray, which can be set to display either your CPU temperature or your mainboard temperature. If the temperature reaches certain thresholds (which can be set from the configuration dialog box), the number will change from black to yellow/red. The program can also be set to sound an audible alarm if the temperature reaches crtical levels.

More information can be found at www.hmonitor.com
The program costs $29, and a free 30 day trial is available.