I thought it was originally to be called Windows 2000
No, Windows 2000 was called Windows 2000. Something to do with the year it was released in, I believe.
XP was released in 2001, so Windows 2000 would hardly have been an appropriate name for it.
Of course, many people did say that XP was what Win2K should have been, rather like Win 7 and Vista, so perhaps that's where your confusion originates.
FBW