Sorry, I was away for the weekend.
Back to the MBR. The MBR is created when you partition a disk. You said "GParted shows -- as expected -- the empty partition = not allocated." which means there is something (a partition ?) on the disk.
That would explain why the XP install disk doesn't seem to "see" the hard disk - if you are expecting the XP install to give you the option to partition the disk and it doesn't, it's because the XP install is not detecting a hard drive with no partitions.
I mentioned DBAN before, but that is the nuclear option. You could try using the MiniTool bootable partition manager first though. The download site is
http://www.partitionwizard.com/partition-wizard-bootable-cd.html and although I have not used it, it loos as though it will do what you need to do and an earlier version got an excellent review from a site I trust. As with DBAN, you will need to download it, write a CD and boot from the CD to use it.