I suggest getting another fan, if the existing fan fails to cool it properly. Did you add a videocard or 7200 rpm HDD lately? Those are also heat generators.

Is your fan spinning? Does it makes a weird noise? Is there dust blocking air vents?
While I did computerrepair, I saw portable's with sandwiches inside. Desktops with sand in them. Desktops with coffee and sucker inside... just to say look inside what's happening. Look to your heatsink, is it still attached properly, does it make proper contact with your cpu?