Click start>programs>administrative tools>computer management. Under storage click disk management. In the top window, right click the drive you want to change and select change drive letter and path. Click edit and there you have it.
Win2K won't let you change drive letters for system(boot) or start (Win2K system) drives. It happened to me that I have a dual WinME/win2K installation and I wanted drive C: to be C: both in WinME and Win2K, but for some reason W2k saw this drive as E: and wouldn't let me change the letter since it was my boot drive. I discovered that you can go to the Registry to HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices and edit the entry named \DosDevices\(DriveLetter): substituting it for the letter that you want. After that you can reboot the computer and it works. Only make sure that you don't have repeated letter assignments, only God knows what could happen in this case.