Following link will shows you how to use both languages in Windows XP but I don't think you could completely change it to English XP without reinstalling the OS:
http://greggman.com/japan/xp-ime/xp-ime.htm
In regards to Japanese Version Windows XP SP2, I have a copy that I could let you use (After all it is not illegal to share the Service Pack, I THINK) , anyways, It's about 470 Meg ISO Image, if you have and FTP account somewhere then send me the info via PM and I will upload it for you.
Cheers