It'll take more than a reformat if the Partition or drive used to have Vista on it. You have to delete the partition (if there is one) and remake it. This will totally remove Vista's version of NTFS. Do this with the Windows XP installation disk. When it boots up and asks you where to install windows use it's partition/disk tools to remove the partition or fdisk the drive.
That should sort it.
That should sort it.