In order to return to XP you are going to need to do a clean install of XP.
Your PC should have either a restore partition and\or restore CD's that came
with it.  I would consult the user manual or the computer manufacturer's web
site for specific instruction for your machine.  Before doing the clean
install of XP make sure you back-up any data you wish to save first.  A
clean install will wipe everything off your drive so if you don't back-up
you'll lose all your existing data..

Hi Billie,

It wasn't meant to be a "trial" as there is no uninstalling an NT system
upgrade. You have two options:

1) If the windows.old folder exists on the root of the drive, follow the
steps here:
http://support.microsoft.com/kb/933168

If it fails to work, read #2:

2) If it doesn't, you will need to save important data to removable media
and start from scratch. Do not just format the installation drive, you will
need to remove and rebuild the system volume as part of setup as XP will not
be able to install to a volume that was altered by Vista (minor variations
in how ntfs is implemented by each of the OS's preclude this).

Another nasty surprise the clueless fanboy monkey squad never want to
talk about. Once you put Vista on your PC you screwed it up for prior
versions of Windows IF you attempt to reuse the same partition.

Why?

Vista requires a slightly different flavor of the NTFS (file system)
that isn't compatible with XP. Talk about a surprise!

The only way to put XP back, is do a clean install which of course
means you'll need to copy your data to some other partition, and
external drive or to CD's, DVDs first and then you'll need to
reinstall XP from scratch along with and all your working
applications. A real bumber.

    After backing up any data you wish to transfer to the new OS
installation, simply boot from the WinXP installation CD.  You'll be
offered the opportunity to delete, create, and format partitions as part
of the installation process. (You may need to re-arrange the order of
boot devices in the PC's BIOS to boot from the CD.)

HOW TO Install Windows XP
http://support.microsoft.com/default.aspx?scid=KB;en-us;316941

http://www.michaelstevenstech.com/cleanxpinstall.html

http://www.webtree.ca/windowsxp/clean_install.htm