In a CIS 101 course the other day, the question came up about the fans inside the system unit/tower. The fans are there to cool your system, so if they aren't running, it may be indicative of a problem.
When it is not a problem? Well, some fans are designed to be "green" - so in other words, they may shut off once in a while if they are not needed, in order to save you power.
SpeedFan will let you (on a basic level) check temperatures inside your computer, and (on a more advanced level) view some of the diagnostic readings from the hard drive and even turn your fans on more or less often.
I don't recommend that option for beginning users.
Anyway, this free software can access lots of different pieces of information. I have two hard drives, and it was able to tell me that the temperatures were 36 degrees Celsius in one, and 48 degrees Celsius in another. A little Web research told me that these are well within acceptable ranges, so my computer is okay. Note that if you start seeing numbers like 60 degrees Celsius, this is a bad thing.
So, if you are wondering how hot it is inside your PC, this is one utility that you can download and use to check it. If you're someone who is looking for a good diagnostic tool to use on other people's machines, this is a good example of a free software program to have in your diagnostic kit.