If you can hear the fan running and Dell Diagnostics do not indicate a problem it is possible that the air vents are blocked by "dust bunnies".(provided you do have a heat problem)

1.Check the air vents

2.Check if the heatsink system is properly attached to the CPU - look for loose screws(very rare)

  If the heatsink looks OK you don't have to apply new thermal paste - see note below

3.Download "Speccy" (there are tons of similar programs on the market) install and run it.

  It will show you the actual CPU temperature (40 to 60 degrees should be ok - I am guessing)

  http://www.piriform.com/speccy

  If the temperature is fine it is more likely that you have a software problem......

Note:

Normally one would apply fresh thermal paste if it is necessary to replace the thermal system or parts of it which are directly in contact with the paste.The paste is nothing more than a "gap filler" between 2 uneven metal plates.It contributes to a better heat flow from the CPU to the heatsink as it is a better heat conductor than trapped air between 2 surfaces.

I am not a Dell member - this is just my personal opinion