with vista still fully installed, boot to your XP installation
go to start -> run and type "c:\boot\fixntfs -xp" (without the quotes) click ok, and reboot the system
when the system reboots you should notice the bootloader has gone
If possible, move the files from the D drive that you want to keep to C for now.
boot back into xp again and go to disk management
select the drive you have vista installed on, right click it and select format.
then, go to windows explorer and delete these files from the root of your xp drive or partition:
boot (folder)
boot.bak
bootmgr
bootsect.bak
right click on 'My Computer',
select properties -> advanced -> startup and recovery settings, and under system startup, make sure that windows xp is selected as the default
If you don't have the fixntfs file, I have attached it to this post.
After doing all this I still had some problems with booting so what i done was to insert my xp disk and go to the recovery console and use the Fixboot c: command to get rid of the last bits of Vista and get XP booting on its own again.
No data will be lost on your c: drive and your current xp configuration with software installed and files saved will not be altered at all.