|You can easily get XP to boot from what is now your D by running bootcfg /rebuild in the Recovery Console, and you might as well run fixboot and fixmbr while you're at it.|
Of course, the bios Setup has to have the D partition set as before any other hard drive partition in the boot order, if it lists D.
I don't know what you are talking about regarding changing the drive letters in the registry, but I do know in Disk Management you can't change the drive letter of the partition Windows is installed on - you can change any drive letter but that one. Another thing you can't do there is assign a drive letter that is already assigned. It's quite possible if you try to do either of those things in the Registry, Windows won't let it "stick".
If the drive letter you want to use is already assigned, you have to change the drive letter of that partition to something else to free up the drive letter you want, then later change the drive letter to the one you want since that drive letter is then available.
Insert Windows CD, start computer, press a key to boot with it, go to the Recovery Console
8. At the command prompt, type: bootcfg /rebuild, and then press ENTER. This command scans the hard disks of the computer for Windows XP, Microsoft Windows 2000, or Microsoft Windows NT installations, and then displays the results.
Follow the instructions that appear on the screen to add the Windows installations to the Boot.ini file.
For example, follow these steps to add a Windows XP installation to the Boot.ini file:
a. When you receive a message that is similar to the following message, press Y:
Total Identified Windows Installs: 1
Add installation to boot list? (Yes/No/All)
b. You receive a message that is similar to the following message:
Enter Load Identifier (my note - type something, anything, or you will see no label when you boot)
This is normally the name of the operating system. When you receive this message, type the name of your operating system, and then press ENTER. This is either Microsoft Windows XP Professional or Microsoft Windows XP Home Edition.
c. You receive a message that is similar to the following:
Enter OS Load options
When you receive this message, for 2000 or XP, type: /fastdetect, and then press ENTER.
Note The instructions that appear on your screen may be different, depending on the configuration of your computer.
9. Type exit, and then press ENTER to quit Recovery Console. Your computer restarts, and the updated boot list appears when you receive the "Please select the operating system to start" message.
Bootcfg does not delete the existing boot.ini - it adds new entries that are listed before (above) the older ones.
You can remove the old entries, at least one of which may now be invalid.
E.g. Control Panel - System - Advanced - Startup and Recovery - Edit - maximize the window - delete the entire lines below the lines below [operating systems] that were not detected by bootcfg.
Choose File in the top left corner - Save.
If after editing boot.ini there is now only one operating system listed after [operating systems] in boot.ini, after you reboot, your computer will now boot without prompting you to select operating systems.