One cannot change the language version of Windows XP.
You must purchase the "Full English Version" of Windows XP,
reformat your drive and install it "clean". The alternative is to
create a dual-boot configuration which requires an available,
empty partition. You can purchase and use Partition Magic 8
(www.powerquest.com) to accomplish this.
If you have a desktop computer, or a notebook computer that
will accept a second hard drive, you can install a second hard
drive and install the "Full English Version" on it and end up
with a dual language boot configuration.