As already explained to you, the simple fix is to re-apply the thermal paste. Then be 100% sure the cpu fan is in 100% contact to do its job, as in level and firm. Don't over-tighten the fan setup though. be sure the proper amount of paste is used, DON"T over do it. Also, recheck to see the cpu fan setup is "rated" for you're cpu, it has to say so as in what level or class of cpu bested used up to.
Instructions for any paste: http://www.arcticsilver.com/instructions.htm
FYI- The 8600 class GPU is prone to heat issues, so it must be cooled well. The case should also looked at for overall cooling aspects. As the PC ages, cooling does degrade alittle so don't fall for it didn't do that before scenario, things do age to wear&tear.