Started recieving this message off and on for a few weeks.  Now the machine will not boot at all.  I receive message F1 to reboot or F2 for setup menu.    Primary hard disk 0 not found.   I cleared NVRAM per Dell instructions.  No change.  I replaced HD with known good drive.  The drive I replaced tested with no problems.   Once again installed the original HD that tested good.  Removed internal battery and let sit for 15 minutes.  Replaced battery and disconnected ALL IDE drive cables, including HD.  Cleard NVRAM again at Setup Menu, (capslocks,numlocks,scrolllocks must be activated, then pressed ALT-E).  Beeps to confirm NVRAM is cleared.  Reconnected IDE cable to HD, powered up.  HD was found.   Reconnected other IDE cables.   PC is booting correctly at the moment.  Hopefully this has resolved my problems.