The components inside your computer, in particular the processor itself, generate a lot of heat. If you look inside you will see at least one if not two little radiators. These are little aluminum finned things that help remove the heat. In a desktop computer, you will also find at least two if not 4 or 5 fans constantly running. They are there to move lots of air past these hot devices. In some cases there are heat sensing devices that can actually increase, then decrease fan speed.
All this air movement draws in dust. The dust gets trapped in the cooling fins. That trapped dust reduces the effectiveness of the cooling system. That causes the devices to run hotter. Eventually, as the problem gets worse, the heat will destroy the devices. So what can you do? Do not block the vents. Change your furnace air filters often. If you must put the computer on the floor, find a way to keep it off the carpet (one of the worst sources).
Every year or two you should blow out the dust from your desktop PC. Shut Windows down normally. Remove the power plug. Hold the on button in for 5 seconds. Open up your desktop computer case with great care and take a look.You may be surprised. Do NOT TOUCH anything inside. Get yourself one of those pressurized cans of air and put the plastic nozzle on it and blow the dust out. Particularly around the finned coolers.
Your notebook PC probably has a fan in it too, but it is impossible to actually see it and very difficult to blow it out. I do not recommend this.