The most common sluggish HDD item I encounter is the old XP DMA BUG. Google that and it's a simple procedure with no reboot. I find that many folk won't do this and I won't write what I think about that.
Next is the old indexing. It can be triggered by disk and registry cleaners. Hint: Don't do that.
Finally is your antivirus scanning and scheduled defragment.
Yes you could find PROCESS VIEWER but I find most folk find it too challenging.
how do I find out what my system is doing. Often I have hard drive activity which slows or stops me from doing anything. How can I find out what is causing this?