First of all partition a drive up. Make sure its a large capacity drive, something well over 100GB.
Boot the computer with CD/DVD set as the first boot device and pop the XP
CD in the drive and allow setup to begin. When you come to the screen where you select the drive partition you should see the drive listed with all the space listed as unpartitioned. If the drive was in use previously, you'll need to delete all the existing partitions. To do this you'll have to hit the 'D
' key followed by the 'L
' key. The screen instructions will be visible to guide the process.
Once all the existing partitions are deleted the display will show the total unpartitioned disk space.
Hit the 'C
' key to create a partition. Personally i partion mine to 80GB (81920MB)
On the newly created partition, (not the raw one) install XP on that. Carry on with the installation until its final restart, it will be a light blue screen.
Quickly pop the Vista disk into the drive and start the Vista install. When it askes where you want Vista choose the unallocated partition. If you let XP load up first then you will not have the dual boot option on start up.
If you have done this correctly then each time the computer restarts during Vista install you see a boot message on screen saying "older version of window(XP) and Vista.
Lets Vista fully install right up until it loads the welcome screen. Then you can restart the computer, choose older version of windows to complete and load the XP operating system.
You then have to switch between them both to install drivers, software etc.
If you want to use both operating systems at a time then you need a virtual operating system installed but they are not that good as memory is kept at a minimum.
Good luck and please post back if you need anymore advice.