There's a free program at
http://crystalmark.info/?lang=en which can give you your HD temperatures (I believe you have to have SMART enabled -- this is usually done in your motherboard bios.)
Also, when you render, do you use more than one thread? Maybe trying a min/max of 1/1 will show a difference, since the CPU activity will be much less, hence less heat.
Check your cable routing to see if any are blocking fans. Is your power supply rated high enough for your computer?
Unless your case has excellent airflow characteristics, there's a good chance there are "hot spots" or "dead zones". This is where the air doesn't flow well. This can also cause problems. If you have a small desk fan, you can try opening your case and directing the fan in the general direction of the case airflow (like input air in front, exhaust air at back) so direct the fan so it tends to blow in the front-to-back direction. Also monitor the temperatures to see if the external fan has a significant effect.
My CPU is 38c at idle. A single threaded TG2 render takes it to 48c. A multi-threaded render goes to 57c. If I direct a small fan into the open case, the temperature of a multi-threaded render goes to 46c, so that external fan has a real impact. I use the external fan whenever I'm not able to monitor my computer during a long render (like sleeping.)
Of course the good/bad CPU temperature is dependent on the type (of CPU). I have older hardware. But even if your CPU can handle higher temperatures, the extra heat will effect other parts of your system (HD, Memory, GPU, etc.) And the ambient room temperature will make a big difference.
Anyway, with all the inputs from the participants of this thread, you should be able to safely and inexpensively rule out potential problem causes. And as mentioned, it's very difficult to troubleshoot over the Internet.
