Unfortunately it's not a simple process:
1.) Backup all of your files.
2.) Wipe your hard drive clean.
3.) Use the Windows XP CD to create two partitions on the drive.
4.) Install Windows XP onto the first partition.
5.) Install Vista onto the second partition.
6.) Reinstall all of your programs and replace all of your files from backup.
P.S. You can also read another member's (Yoni's) reply by clicking here.
i have windows vista but i like to know how to install windows xp in my computer running windows vista, in the same hard drive. how can i do that? thank you for your help.