What is my computer (hard drive) doing now?- I can hear it working

Open Resource Monitor from Task Manager (or type Resource Monitor in the search box). Look at the activity under the Disk tab.

When you defrag your computer you can hear the hard drive running as it changes files back and forth. At times I can hear the same sound (or similar) but have no clue what it can possibly be doing. My 1st instinct is to unplug my internet connection should someone be hacking in my PC and using my resources. But after doing so I can still hear it working hard, on and on, and on at times. So I do a Ctrl+Alt+Del to see what is using most of my memory. Often its Firefox and Skype but even after closing both and doing nothing (hand away from keyboard and mouse) I can still hear it at work. I do not have any automatic updates for any of my software so it can't be that. I was wondering if there is some way, software perhaps, to find out exactly what it is that is making my computer do such a thing?

It's probably defragging. There's no need to do it yourself; Windows does it automatically on a schedule.

Go to Resource Manager not Task Manager. On the Disk tab, click on the top of the total columns to sort them so you can see which process is doing most I/O and which files are being accessed the most. Post the results here or upload a screen-shot. Someone should be able to tell you what they are.

CTRL + Shift + Esc > Task Manager > Processes > Ensure that you have ALL processes showing > CPU header > click on it until all high numbers are grouped at the top..

Make a note of the processes with the highest CPU value and then look up each one to see what it does.

There is also a strong possibility that your hard drive is on its way west. Noisy drives are not happy drives. Windows may be continually shuffling stuff around in a bid to get it all onto good clusters..

Thanks to you as well, dax, but as I said, I already looked at the task manager (that's what ctrl+alt+alt does) and though the info may be there, it is not specific enough for me to understand what it is exactly that is doing such a thing- and that is my question. What I wanted to know.