Fdisk has no knowledge of drive letters.
The OS assigns drive letters - it will assign C to the primary partition marked as "Active" on the first hard disk (primary Master on IDE systems).
The OS will (usually) then assign drive letters sequentially first to detected fixed disk partitions, then to removable drives. That is the default behaviour, though it is possible to re-assign drive letters to all but the A, B and C drives (again, there can be exceptions) in Windows 9x and NT+.
Windows NT 4.0, Windows 2000, Windows Server 2003, and Windows XP Professional define the “system” and “boot” partitions differently from other operating systems (typical MS). The system volume contains files that are needed to start Windows XP Professional, i.e. Ntldr, Boot.ini and NTdetect.com (other files may also be required for multi-boot systems or SCSI disks - these are the bare minimum).
The boot volume contains Windows XP Professional operating system files and folders such as systemroot and systemroot\System32. The boot volume can be, but does not have to be, the same volume as the system volume.
Therefore it is possible to have Windows "installed" on ANY drive "letter", as long as Ntldr, Boot.ini and NTdetect.com are on the System Volume - i.e. primary active partition.
Confused?
SD