Check out this great post from the Windows Server Setup / Core Team, which details the WinSxS folder - the single-instance file storage location for all system files (components) in Vista and 2008. Fascinating stuff.
http://blogs.technet.com/askcore/archive/2008/09/17/what-is-the-winsxs-directory-in-windows-2008-and-windows-vista-and-why-is-it-so-large.aspx