Just boot from your XP CD. When you reach the screen that lists the current partition(s), delete the one that contains Vista (select it, press D, press ENTER, press L)and then press C to create a new partition to install XP on. Using DOS probably won't work because Vista comes on large hard drives that use the NTFS filing system which is unsupported by DOS.
Re: MS-DOS Boot Disk Contents
It's actually rather simple.
Just boot from your XP CD. When you reach the screen that lists the current partition(s), delete the one that contains Vista (select it, press D, press ENTER, press L)and then press C to create a new partition to install XP on. Using DOS probably won't work because Vista comes on large hard drives that use the NTFS filing system which is unsupported by DOS.