If it is an old PC you should clean off the thermal paste from the CPU and the heatsink and apply a new thin layer of thermal paste. I like Arctic Silver 5, it used to be king since there are better thermal paste around now, but AS 5 is still pretty damn good.
Thermal paste can dry out over time. I once tried out the thermal paste that came with Tuniq Tower (TX-1 or MX-1???), after a year and a half I noticed my idle temps hovering at about 70C!!!. The damn thermal paste dried out and basically turned into a clump of powder, which by the way makes a good heat insulator because the pockets of air prevented good thermal transfer. I quickly applied my old 'ol AS 5 and the temps dropped back down to around 36C - 39C when idling.