Clearing data left by fdisk

After reformating a HD, Windows won’t install a new OS because the OS from the old partition is still readable enough that it sometimes tries to load at bootup. It can’t be found on any drive in DOS on the new partition or load anything that functions. System Commander is the boot mngt. Data read at ‘boot’ differs allot. Is it corrupt MBS, HD, other. I tried instructions for low level formating with DOS codes. I’ll add 100 points (“Cool info”=4 digit #) for any related area (bios,etc) I master with the one or two key bits of knowledge I’m missing. Info that makes it work=points until HD lying on desk goes in (week?) then?