The issue is that I found almost 8M space in original hard disk was reserved if buying a new Dell desktop. When I tried to install a new OS like Windows XP, the hard disk must be firstly formatted, then I could see the status of all current partitions, apparently, there are always 8M size partition reserved by default in the hard disk. I don't know why the original hard disk always is reserved for 8M remaining space in Dell machine.
Does anyone know it? Thanks in advance.