While many were unaware of the need to clean heatsinks and check fans, some machines had failing motherboard capacitors which I had the displeasure of finding in our office desktops. These caused increased power consumption and excessive heating. Link with pictures at http://www.badcaps.net/
After cleaning, there is a non-PC-Hardware issue of spyware and pests. Some will cause a machine to run at full tilt. Today I suggest we scan with EWIDO and HOUSECALL (google.com please) and toss what those scans find.
My Dell Pentium (4 years old) is constantly running the fan and putting out hot air.