Consider this gist to be deprecated. It's full of information that may or may not be correct, given recent findings. Consider the in-development PIDDatabase, and UMSKT, instead: https://umskt.github.io/PIDDatabase-viewer https://github.com/UMSKT/UMSKT
These keys have been tested to work. These are a combination of keys from Chinese websites, trial keys inside ISO's, auto-activate keys in OEM ISO's, and directly from Microsoft's website, all aggregated for your convenience.
In order to use these keys, you need the right edition of Windows XP/2003. Not just Home/Pro/Enteprise/etc, whether it's a Retail, OEM or Volume version. There's a clear cut way to check this.