Yeah, I agree, I dont think even Fdisk can access an NTFS partition, I think (may be i'm mistaken) if there's only one partition and its an NTFS partition, then Fdisk won't recognise that partition at all, as Fdisk is based in DOS.
In the original question, for what reason did you want to boot to DOS, may be to format the drive? To do that, I'd throw in my XP cd and start to go through the install routine, in one of the screens it gives you an option to create/delete any existing partitions. So if you wanted to wipe your drive, just go through the install routine from the CD.
Hope that helps