BIOS HDD changing without intervention

I have sent out a number of units with windows xp home, with ASUS, GIGABYTE, PC100, motherboards.
After a couple of months I have received calls about not being able to boot the computer.
I had each check the CMOS setting for HDD, and each indicated it was set to AUTO. When I shipped these units I set the HDD to user and setting for the particular drive specs, (MAXTOR).
They all had NORTONs 2001, 2002 installed aand active.
Can anybody shed any light as to a culprit that would do this change in the CMOS.
The clients say they did not enter the CMOS…

Thx