In safe mode all the drivers are not going to tap into the full potential of the machine. Your disk transfers are slower, the graphics card and CPU don't work too hard. This results in less heat, less draw on the power supply and more. Sort of like driving your car at 50 MPH. It's very unlikely to overheat.
It's a good question and one that I wish was written up at length but this is all typed by me so I have to keep it short.
Read http://forums.cnet.com/7723-6122_102-569818/getting-ready-to/?tag=contentBody;threadListing to see another member dive in and clean up their PC.