Você tem que formatar a unidade, porque o CD de inicialização do Windows XP não reconhece sua partição do Windows Vista.
(Isso me levou a apagar acidentalmente meu disco externo em vez do interno)
Se a sua placa-mãe inicializar a partir do USB, você pode usar o Active @ Kill Disk para limpar o seu disco e instalar o Windows XP nele.
Active@ KillDisk - Hard Drive Eraser is powerful and compact software that allows you to destroy all data on hard and floppy drives completely, excluding any possibility of future recovery of deleted files and folders. It's a hard drive and partition eraser utility.
If you use FDISK, FORMAT utilities, or DELETE standard operating system command for data removal, there is always a chance to recover deleted files (using undelete utility or some data recovery software) and use against the owner's will. We highly recommend you to run this FREE utility for the hard and floppy drives you want to dispose of, recycle, re-use, sell or donate to somebody.