Hard drive always being accessed?

Hey guys... i was wondering what programs (if any) would constantly access the hard drive? I disabled hard drive indexing and have also turned off antivirus seeing if those were causing it but it still continues to be accessed and the light blinks like im reading the HD. I have also ran anti-viruse, anti spyware scans and defragged. Is this something to be worried about?? Its a 160gig HD (not sure on brand) running XP.