HDD Removed from Bios

My system is based on a ECS K7S5A+Athlon1000+SDRAM 256Meg+WD40G HDD+ WD80G HDD+CD+CDR+GEFORCE2MMX running on W2K. My son got a lot of hang up by running games on the net but this week-end, the system hangs during a DiabloII save and impossible to restart: after investigation, both HDD drives disappeared from BIOS setup. I was able to have them back active in the BIOS, boot correctly backup my system from one drive to another but the same bug appears after a while. Any idea?