When you upgrade your computer from Windows 95 to Windows 2000, drive letter assignments for unformatted partitions may not be migrated.
To work around this issue, it is recommended that you format all drives prior to installing Windows 2000 if you want to preserve your current drive letter configuration.
This behavior is by design.
For additional information, see the following article or articles in the
Microsoft Knowledge Base:
How Windows 2000 Assigns, Reserves and Stores Drive Letters