I just tried it and it worked.
Maybe you have a virus?
When installing SP1A, I get a thing that says the server will not respond.
That is what MS says about no support for XP anymore.
How to obtain the latest Windows XP service pack
Everything is on this page (SP 1a, 2, & 3).
You might want to burn a disc of all of the service packs for future reference . . .