I left my comp for a while and when I came back the HDD acsess light was on steadily (no blinking, ect). I noticed this on my laptop too. What is windows doing?
Either that or it could be Windows Restore, Update, Antivirus, Firewall or anything like that.
I'd just run an adware program and if nothing comes up then your safe so don't worry about it. You could also run a virus check as well just to make sure.