If you have a Technet subscription, you can get early access to Windows XP Service Pack 3 from the Technet Subscriptions page. Scroll down the page and look for the grey “Top Downloads” section.
I’ve been running it for a few days now and haven’t had any problems 🙂
If you don’t have a Microsoft Technet subscription – which I highly recommend – you can probably find a direct download link around somewhere.