Well, you can try the download link under the All Products button on Microsoft's web site.
CAUTION: There are a couple of security fixes that are ONLY available in SP1. There are 3rd party solutions posted at a few XP support web sites.
I wonderabout your product key.
One of the 'features' of SP1 is a product key test that disallows downloading updates for known 'pirated' product keys.
If you are certain that your Product key is valid, you should contact Microsoft to find out why it isbeing rejected.
As to the automated running of the XP update Wizard, you need to disable automatic updates, or remove the Wizard from the list of startup items.
Chas