You're off topic for this forum, which is dedicated to Windows Live OneCare. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
As far as I know, the themes from Windows XP are not downloadable from Microsoft. You may be able to locate them on the web by searching. There are many 3rd party theme sites. You may also be able to recover the files by copying them from another XP machine.
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator