My wife’s PC used to drive me mad! Every time I sat in the office I could hear the fan running at full tilt, especially after she had played Facebook games and left them running.
Something had to be done!
A quick check showed high CPU use and the temperature (checked using Speccy) well above normal.
My first reaction was that she needs a new PC but my frugal (cheap) side kicked in and forced me to dig a little deeper so I removed the PC cover and discovered that the heat sink above the CPU was entirely buried in fluff!
30 seconds with a vacuum cleaner and the office is back to its former library like state. Ahhh!