Fdisk won't work from any old bootable floppy! It has to be a floppy with the fdisk tool on it, which ISN'T what the xp bootable floppy is.
To run fdisk, you need a win98 startup disk, which you can create by downloading a win98 boot disk creator (I forget the address I got one from!)
If you only want to boot to DOS to run a utility from a disk, then just create a bootable disk in XP by rightclicking on the a: drive and selecting format, then checking the bootable disk option. After you've created it, ad the utility to it. This is only any good though if the disk you want to run the utility on is FAT32, as the XP bootable floppy can't read NTFS.