someone has attempted to re-install XP but has 'almost' created a new partition with XP installed on that instead. Only you know the history of this machine, so I can't say.

You will need to edit XP's Boot.ini file, but be careful. Make a mistake and you can lock yourself out of the only working XP.

Microsoft -How to edit the boot.ini file in XP.

There's more practical advice here;
(Note the very last post).

Good luck, and make sure you do the right thing.