HDD not recognized @ POST

The 20gig HDD not recognized at power on. Says “non system disk in A” Restarting with the reset button boots up XP Home OK. Tried fixmbr and fixboot from repair console, no help. Also, tried different bios settings, no help. Checked the ribbon cables, ok. The computer is a dell dimension 4100 w/384m ram, p3 900mhz.
Any suggestions are appreciated. Thanks.