Hard drive Constantly being read

I am sorry If this thread already exist, I attempted searching for it and the search yielded no results.

So as you can judge by my sig, I built a very decent gaming rig, it used to be my main rig where I conducted much of my school work and web surfing but now that has become my gaming only rig and my Internet and email machine is what im on right now, my Acer Aspire One.

So the problem is with my gaming rig, I don't know for how long its been going on but I would turn on my rig and just leave it there. however, I would walk away and go do other stuff and forget it was on, but I come to check up on it always (idk why) and I always see that the Hard drive light is always going, and I haven't done anything! 20, even 30 mins after the PC has been turned on it is still reading or doing something to the HDD. I have antivirus software up so i doubt its a virus or anything like that. I've gone to MSCONFIG and done the whole cleanup thing to turn off all unnecessary services and startup items and the issue persists.

I am running Vista 64-bit and gone through the Task Manager and the performance tool and I don't really see any weird processes being processed but what is weird is that I see it reading some of my CoD4 files (like iw_06 or iw_07) and i haven't even touched the game.

So any ideas on why this might be? I haven;'t done any upgrades or anything like that, its the same rig since its it was built.