Sounds like a bios problem.
Try checing the boot order in the bios and make sure the Hard Disk is first ot second and that LAN boot is not included in the boot order list.
How old is the PC? (it may be the motherboard battery)
How many Hard Drives do you have?
What make/model is the Hard Drive (and is it SATA)?
Finnally or perhaps first check there are no CD's in the drives.