Firstly, as I am typing this on an XW6400, I salute you. They made computers properly back then.
IMHO, trying to fix a copy of XP that's been messed about with beyond a certain point becomes not only economically not viable, but just unfeasible, especially when dealing with crucial guts of the OS. Even if it starts to work again, it'll never be quite the same.
If you install XP again right on top of the existing installation, it should keep your documents / stuff, while working properly. You will probably have to re-install your programs though.
Activation using the automated telephone system invariably works, if you lie at the relevant prompt. Good luck!