Everyone is describing this to you:
307844 How to change drive letter assignments in Windows XP
http://support.microsoft.com/?id=307844
It's not going to work. Won't work for your boot drive. You'll need to use this although I don't recommend it.
223188 How To Restore the System/Boot Drive Letter in Windows
http://support.microsoft.com/?id=223188
Careful with that! If you've installed a lot of software you're going to have problems if they make any references to your drive letter. I would highly recommend a system restore point before you begin. You'll also want to familiarize yourself with how to change your userinit.exe path:
249321 Unable to Log on if the Boot Partition Drive Letter Has Changed
http://support.microsoft.com/?id=249321
This should help you figure out how you ended up with the wrong letter in the first place:
234048 How Windows 2000 Assigns, Reserves, and Stores Drive Letters
http://support.microsoft.com/?id=234048
If you are not too far into your move-in I would restart.
Edit: mmm, yeah scratch that. Just noticed you are fooling with a data drive letter not OS. Disk manager should do it. D'oh.