op never said anything about 100C.....high processor usage is generally due to too many applications running in background. open up the task manager and see what process or program is using the most amount of process,and if you feel that you do not need that,close it down.
many a times, malwares also lead to high processor usage because they constantly run in the background,so do check for malwares too.
your heating issue is due to high processor usage and since your laptop is quite old,you will surely have a lot of dust and debris on the air vents. have them thoroughly cleaned by an expert if you cannot do it yourself.
Agreed. Some more ram wouldn't hurt either.