Let me recap:
Only 1 disk.
C is listed first (on the left). It is light blue and has a green frame. So it's a logical partition.
D is dark blue and no green border. So it's a primary partition.
No other partitions shown in disk management? Sure!!?
If so... D is the boot partition, because only primary partition can be active.
In gparted that partition is flagged as "boot".... correct? you also have an "extended" partition. Within that is WINXP