Something accesses the hard disk every two sec (longish)

Hi,
I was given an old Compaq Deskpro with a 350Mhz PII and 6 G Maxtor hard
drive, from which I removed the vestiges of NT4 and installed Win98.
It worked well enough with 196 M of PC100 RAM
But something regularly accesses the hard disk every two seconds or so,
like a clock or heart beat. And I just dont know what it is. The previous
user says it always did this, so I don't think it s anything acting thru
the Windows OS which is causing it.
I did not think this regular accessing of the disk mattered -- it simply
meant a little clicking noise and a blink of the led. Or so I thought.
Until after setting the puter up for a friend. I got into a bad series of
install/uninstalls of dodgy bloatware. After which I wanted to clean things
up a bit with Scandisk and Defragmenter.
It then transpired that the disk acessing would not allow Scandisk or
Defrag to get under way and complete.
What could be accessing the disk? Could it be coming from the BIOS? (One
of the first things I checked was that the MSWord feature FastFind was
indeed disabled and closed, so I dont think its that.)
I would be grateful for any thoughts on this one.
Michael Anders