Hard drive constantly busy

This Win98 machine recently rebuilt(128MB RAM – 14G HD presents with a constantly active hard drive. Even when no applications are running the hard drive remains busy. The machine is relatively stable was a ‘clean’ install, evinced no driver problems, and to all extents and purposes appears normal, and behaved well for about two weeks after rebuild. No unsual applications seem to be running in the background (MSCONFIG). Does anyone have any pointers that might assist me in determining the cause, prior to formatting and a second rebuild. Thank you in advance.
JdeP