Well your system spec should be up the task. Though I suspect this is a laptop (?) given the intel i5 2.5GHz, which are more susceptible to heat issues. You could check the air intakes/exhausts and ensure they are clean/clear.
But as avast is an on-access scanner, whatever you are doing (which you don't mention) is going to have avast scan that activity. This is going to increase CPU activity and correspondingly heat if it is prolonged activity.
What Temps and CPU % activity (for avast services) are you seeing ?