Might depend on what version of Vista & whether its OEM, and if its a new PC
whether winxp drivers are avilable for the hw

You mean a version of the Windows Vista Operating System to a version of the
Windows XP Operating System?

It would require wiping the computer - starting with a completely clean
slate - installing the new (to the computer) operating system (in this
case - Windows XP of some version) and then installing the proper hardware
drivers from each of the hardware vendors for each component of the computer
so that the operating system (Windows XP) could communicate and utilize each
of the hardware components properly.

Hardware components that usually need drivers include - but are not limited
to - the motherboard chipset, the video card, the sound card, the network
card, the hard disk drive controller, the CD/DVD drives, the printers and
scanners you might hook to the machine, some digital cameras, web cams, some
mice and keyboards (mostly those with extra features) and so on.

Then you could install any software you have licensing for and installation
media/product installation serial numbers/keys for that would function on
Windows XP as opposed to just running on Windows Vista.

So what is required of you?

- An unused license of Windows XP (Home, Professional, Media Center, etc.)
- The installation media for the licensed version of Windows XP and all
other software you intend to install after Windows XP is installed.  (Also
the product keys/serial numbers/etc for each of those where required.)
- The hardware drivers for each component of your computer (internal and
external) for the new operating system (Windows XP in this case) from each
of the hardware manufacturer's support web pages (more than likely.)
Sometimes you can get luck and the manufacturer is someone like
Dell/HP/IBM - and most of the hardware drivers you need can be found in one
place...
- Backups of all of your important stuff...  Internet favorites, pictures,
emails, contacts, documents, spreadsheets, installation media, product keys,
etc and so on.

Then - go for it.

** Not all manufacturers for all products support Windows XP.  Check that
yours does.  That goes for EACH hardware component.