First, let me state that you shouldn't do it. It's a very bad idea that will almost certainly mess up your Windows installation. There is nothing magic about C:, it's simply traditionally been the letter MS operating systems use for the first partition on the first hard drive. There is NO downside to breaking the tradition and having your system volume use a different drive letter.
That said, if you still want to do it, the following procedure is rumoured to work some of the time:
1) Change the drive letter according to
instructions here.
2) Without rebooting, search through the registry for any pathnames containing the old drive letter ("D:\") and change them to the new one. You may also wish to search your hard drive for any application .INI files and correct paths contained in those, but that shouldn't be critical for Windows itself.
3) Pray to whatever deity you worship and reboot.
4) If you have an unusable system, proceed with a
repair instal. If you only have some unusable applications, reinstall them.
The only time I've done this was years ago, on an NT4 machine. It worked fine that time.
P.S. Due to the way Windows 2000/XP installs its bootloader, there is a good chance your system will become unbootable after you kill or format your C: partition. If that happens, you should also proceed with the repair install.
