Grillin'

My PC fan is running on HI all the time. The temperature thing on my case says 38C degrees. It used to hover around 27C when I wasn't playing Skyrim or something. I'm just surfing the net not doing anything laborious. The case has an adjustable fan, it's on HI, the vent's are clear. I don't know why it's so hot. I do have two hard drives as of a couple months ago, but one is just back up it's never doing anything. What could be the cause?