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.