You'll have to check if the heatsink on CPU is full of dust (if it's not cleaned at all for some time you'll have a dust carpet between cooler and heatsink most likely), and if your computer has worked for quite some time it probably needs thermal grease replacement since old thermal grease got old and got indurated. Most of the dust in PC housing can be cleaned with compressed air in can or careful use of vacuum cleaner, but cleaning CPU and reapplying thermal grease needs to be done carefully. You have some videos on YouTube etc. Of all these videos I've found this as a most convenient
http://youtu.be/fRRWbQUqW1Y
I use regular thermal compound most of the times. People say the best is the one with silver component in it, but somehow I don't like it because it gets too messy if you're not careful.
Just to mention that you turn off your PC completely out of the wall socket before doing anything, and if you're unsure about something try to find some help.