The main issue when installing Vista and XP in dual boot is when you have to reinstall any of the two when something fails, let's say a hard disk failure, a lost partition, virus, etc. What commonly happens is that you lose all your hard work configuring and setting your programs the way you like them.
One may spend a whole day or maybe more two configuring, installing and customizing the two operating systems being a gamer, designer, musician, office, internet or just entertainment user.
Do not have to reinstall both OS when one of them fails or if you wish to reinstall any of them (XP or Vista).
Being able to backup each system state separated.
Keep your applications on the other OS when reinstalling any of the two.
Can install any other OS on any other partition you have, for instance: XP, XP 64bits, Vista, Vista 64bits.
If you want to get rid of any of any of the OS installed, set the other as active partition, reboot and delete the desired partition (Remember to keep a backup just in case).
Hard work, since you have to install applications separated.
"Mark partition as Active" every time you have to work on the other OS.
When you lose the Active OS, you have to recover or reinstall immediately if you need to enter the other.
This is a step by step guide on how to do it.
1. Getting Ready
Have 2 partitions on a single or multiple disks, a C:\ and a D:\ for example.
2. Install Windows XP
Install Windows XP any edition on any of the free partitions.
3. Customize your first installation
Prepare and install all your programs the way you are use to do.
4. Prepare the second installation
Once you have your XP installation ready, go into "Computer Management", open "Disk Management", on the free partition, i.e. D:\, right click and select "Mark Partition as Active".
5. Install Windows Vista
Once you have restarted the computer insert the Windows Vista Disk and install on the active partition.
6. Customize Windows Vista Installation
After you install Windows Vista completely, configure and install applications and software, as well as customize your OS as Windows XP.
7. Get ready to swap
When you are ready to swap, just go in "Computer Management" and under "Disk Management" and Mark as active the partition you wish to boot from.
The main benefit is that if you want to get rid of any of them, set the other as active partition, reboot and delete the other partition... for this you don't have to reinstall both in case something goes wrong...