Legal Vista is supplied on dvd with a key to activate it. There are no free legal copies available to download. That's why you can't validate it.
I hope your XP cd is legal and you have a key for it, otherwise you will have the same problem with XP.
You will need to format the drive to install XP, how will depend on whether it's a sata or pata drive, or if it's a brand name pc with a recovery partition.