When getting the error:
A hard disk with UUID {55b773b8-cbcb-42ea-9a44-1368b30ed823} or with the same properties ('D:\VM\VirtualBox\XP SP3\XP SP3.vdi') is already registered.
The error can be resolved by using the utility:
"C:\Program Files\Oracle\VirtualBox\VBoxManage.exe" internalcommands sethduuid "D:\VM\VirtualBox\XP SP3\XP SP3.vdi"