The location of the themes is C:\windows\Resources\Themes. Just remember that you need to replace your uxtheme.dll file (which is protected by Windows) if you wish to use third-party unsigned themes and msstyles. The best way of doing this is using the free third-party multipatcher available here.
Hope this helps,
Windos XP has some desktop themes in it by default. I want to know these are located in which system file/folder?