No one seems to have beaten this yet without formatting and starting again.
I wonder if you could stop XP pro CD seeing XP home then it should allow you to install (just make sure you install to the D partition to prevent overwriting Home.
First make sure you can boot from CD or USB as the following will stop it booting from HDD
Try taking NTLDR and NTDETECT and boot.ini from the route of C: drive, place in a folder ready to go back later, then see if the XP pro disk finds the Home installation. If not then install Pro to D: and edit the boot.ini so that it sees both versions of XP.