ThemeXP.org. The addition of these new servers should help, but if they don't then here is a list of alternative sites where you can find Windows XP themes:
Please note - some sites may contain pop-up advertising.
•Forums at Getskinned.org
(Partial credit for the above list goes to 2kmaro )
If you have any more links to add, or dead links to report, please send the author of this FAQ item an instant message.