The i5-2400 processor has a TCase value of 72.6°Celsius; anything from the Tcase and below will be the expected temperature of the processor in normal use, anything that doesn't stress out the processor like watching movies, burning CDs, browsing the internet, creating documents, etc.
When the processor is stressed out and it is being use at 100%, the temperature will go beyond the Tcase. It can perfectly reach 80 to 90°Celsius and the processor will still be OK. The cooling fan is in charge to keep that temperature there.
If the temperature reaches 100 °Celsius or more, that is consider overheating.