hi frnz,

This is the article which is very helpful for installing xp on already installed windows vista. Vista has an option to get unpartitioned space (shrink volume) option to get xp onto that shrink volume space. What you need to do before shrink volume your active disk space is set your disk space free as much as you can to accomodate your new xp partition.

take backup of all ur data before doing this. Download vistabootpro and put this tool backup too.(free tool)
Note: For hp laptops backup ur swsetup folder in your vista (dont forget). if you do so you can later on install most of your hp drivers from this swsetup folder. xp supports most of the drivers from this folder. just a click away. But some drivers won't so you need to download from hp website you will get it. most of the time you will get problem with sound drivers. Just google it the same with your laptop/desktop model or try the other laptop/desktop model audio drivers i am sure you will be able to find those but need patience and google skills.

once xp installed on your computer, install this vistabootpro in xp ( once u install xp u will not see vista still vista is thr in background , if you set the boot then u will get both vista and xp), set the boot preference from this tool and you are all done.

Here is the best article of what i read before doing the same
cheers!
http://www.howtogeek.com/howto/windows-vista/install-windows-xp-on-your-pre-installed-windows-vista-computer/