I just had a new computer built with a i5-3570k processor. Speedfan shows the core temps are all around 100C. That seems really hot to me, but the guy who sold me the machine says it's "not even close" to a problem. Should I be worried?
The temperature for the i5-3570K should not stay higher than 67.4°C as you can find here:
Processor temperature issues are usually caused by improper fan/heatsink integration or its failure. You may want to remove the heatsink, inspect its pins just to make sure they are not broken or bent and integrate it again, applying new thermal compound if needed. When properly installed, the heatsink should not wiggle or shake if you attempt to move it; the pins should not come up if you try to pull them up.
Make sure there are no obstructions or noises coming from the fan; check the RPMs. The fan should be connected to the motherboard's CPU fan header. Please refer to the following links:
You may also reset the BIOS in your motherboard as an incorrect setting or overclocking the CPU or memory may cause the processor to overheat. The processor should be tested with the default settings and compatible memory. For this step, please refer to your motherboard owner's manual or contact their support line.