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Piggybacking on what RGone said Tomas, if I were you I would redo the thermal paste with some good TIM like Arctic Silver 5. It can be a cheap fix. Also, if you have been running that computer for some time the heatsink could be clogged with dust, thereby reducing its cooling efficiency. Compressed air is the best thing to clean it with but make sure you don't let the fan spin with the air jet as the high speed spin will ruin the bearings.
Every last word "trents" said. If you your cpu cooler is one of those plain aluminum things, I usually stick them in the dishwasher since most canned compressed air will not get all the crud out of those solid aluminum fins. The dishwasher on for just a few mins will clean-R-up.