Your computer is now configured with both versions of Windows XP. Remember
that XP x64 has no 16-bit software support, so any old (compatibility mode)
software you ran under XP previously will not work with the 64-bit version of
the OS. Most games and software should work with x64, though you may have
trouble finding drivers for older hardware.
You are unlikely to see any performance advantage except in games or programs
that have been expressly designed for 64-bit operation. More and more of
these are coming out, though currently the list is restricted to downloadable
(for those who bought original) versions of a few games like Far-Cry.
Windows XP x64 Edition is still in its infancy, but 64-bit hardware support
is a very real fact in today's computer world, and though it usually takes
software (including games) at least 6 months to catch up to what's possible with
cutting-edge hardware, rest assured that sooner or later you are going to be
glad you installed it.
Next, we'll show you how to un-do all of this and return to a single