IDE drives tend to spike the CPU like that for many reasons. Other spikes include having a background image and the list goes on and on without any nefarious cause.
I'm unsure how you can assure someone it's pretty normal until they are exposed to dozens of PCs.
PS. If you want to scan for more, read Grif's post at http://forums.cnet.com/7726-6132_102-5098912.html?tag=posts;msg5099421