mike is correct - alot of the skins floating around are so old they are not compatible with the latest verion of XBMC. If you are downloading the skins from either xbox-skins.net or allxboxskins.com I think you will find that 99.9% of them will not work.
the best place to get the latest working skins is at the xbmc skinning repostiory at:http://xboxmediacenter.svn.sourceforge.net...boxmediacenter/
just grab a SVN client(google on tortoise SVN) and download. They are always being updated to use the latest XBMC features and are compatible with the latest xbmc skinning engine.
hope this helps.