You may have to contact Microsoft on this.
They may want the original disc and issue a replacment,
this would only be to prove your proof of ownership.
I know even trying to find some of the latest service packs to bring a re-install up to speed can be hard to find.
If i remember they were cutting back on the support for service packs for XP as well.
I have XP Pro and had trouble getting SP2 and finally found SP3.
I could not get SP3 to install cleanly though.