good and bad here

Do the bios settings stuff first.

You may have to have the XP disk in hand when deleting the C: root - format it is the best bet but the OS may have boot MBR issues after the deletion or formatting. So you'll need to do the XP disk and first try the Recovery process to add the boot MBR to the current D: root.. BCDEDIT.exe from the command prompt.

Try this if you don't want to format the drive. - locate the free version of EasyBCD 2.0 Install it. Change the default boot to the XP then delete the Vista listing. Reboot the system. Vista should not be there as a boot loader. Then simply delete all files on the Vista drive.

I am not sure if this fails what to do and not do another install to to the XP..

Good luck!