If you don't insist on booting from the XP hard disk (where XP is) itself, you can back up your boot files from C, format C, restore the boot files to C, and continue to boot into XP. This way, the drive letter for the XP drive/partition is unchanged, all programs on XP will work, and is quite simple. You don't even need to open up your computer case and change master/slave settings.
If, however, you want to make computer boot from the XP hard disk, it's slightly more complicated. Plus, the XP partition should (although I'm not absolutely sure) become C, and the registry entries in XP (e.g. referring to D:\Program Files) may all be wrong. Can be overcome, just take a little more work.
If you're content with the first solution, here's my suggestion.
Load XP. In My Computer, click Tools, Folder Options, View tab. (1) Tick "Show hidden files and folders" (2) Untick "Hide file extensions for known file types" (3) Untick "Hide protected operating system files" and say yes to confirm (4) Click OK.
Insert a floppy. Right click Drive A and select Format. On the next window, just tick Start and do not tick any of the boxes. Must do this within XP, and even with a brand new floppy.
After formatting, double click drive C. Copy 4 files: ntldr, ntdetect.com, boot.ini and bootsect.dos to the floppy.
After copying, leave floppy in and restart computer to test the floppy (and go into BIOS to set the boot sequence to be first, floppy, second, CDROM and third, hard disk). If the floppy works, you should get the same old boot menu. Select XP and see if it loads. It should.
If XP loads successfully from floppy, remove floppy, label it "XP boot floppy" and keep it safe.
Next stage, insert XP CD. Restart computer and boot from CD.
On XP setup welcome screen, press R to enter Recovery Console. It should list out your XP installations. There's only one, so it should say 1. D:\WINDOWS
Press 1 and Enter. Enter Administrator password.
At the D:\WINDOWS> prompt, type
format c: /fs:fat32
(one space after the word format, and one space after c: and no spaces thereafter). This formats the C drive in FAT32, because I'm assuming that your XP partition is on FAT32 as well. If your XP drive is on NTFS, then type:
format c: /fs:ntfs
and press Enter. You'll be warned about formatting the C drive, say yes. Wait for it to finish.
AAfter formatting C, insert the XP boot floppy.
Back at the D:\WINDOWS> prompt, type
copy a:\ntldr c:
and press Enter. Then type
copy a:\ntdetect.com c:
and press Enter. Then type
copy a:\boot.ini c:
and press Enter.
These 3 files are all you need now for booting to XP only (no need for bootsect.dos).
and press Enter. Confirm the action. Then type
and press Enter. Computer will restart. Remove CD AND floppy and let computer restart from hard disk. You will still get the old dual boot menu at this stage. Select XP. It should load XP.
Once in XP, click Start, Run, and type:
and press Enter. This opens the boot.ini file in Notepad. Now you want to remove the reference to Windows 98 in the boot menu.
Make sure that the line that begins with the word "default=" is as follows:
This is the path to your XP installation. You want to make sure that it is your "default" choice.
If that's the case, then the only change you need to make is, under the section [operating systems], there is a line that reads like:
C:\="Microsoft Windows 98"
Just delete this line. click File, Save. Exit Notepad. When you next restart, you won't get a boot menu, and the machine will boot straight into XP. Then you can use drive C however you like.
If anything goes wrong, use the boot floppy to boot you back into XP.
If the new setup is working, you can insert boot floppy back in, and delete bootsect.dos, and copy the new boot.ini file from C to the floppy (replacing the one on the floppy).
If you don't like the idea of having your only OS on D while the computer is booting from C, and want the computer to boot from the XP hdd, post back. It's going to be slightly more complicated, but perhaps still manageable.