By wiping the system out and reinstalling everything. Sorry, but there is no
uninstalling Vista, it is a one-way street. To go back to XP requires
starting from scratch.
When you installed, did you do it as an upgrade or by booting the DVD and
installing Vista cleanly to a different partition/volume?
You don't, unless you made a backup before you installed Vista. You can
reinstall XP and all of your old programs, but it won't be exactly the
same as it was, and it could take a lot of time (certainly a half day,
possibly as many as 2 or 3 days).