SharePoint Designer 2010 can only be used to edit SharePoint 2010 websites.
You can't use SPD 2010 to edit regular web sites or previous versions of
SharePoint. This is a change from SPD 2007 because of the changing role of
SharePoint Designer in 2010. SharePoint Server 2010 is the next version of
what is now Microsoft Office SharePoint Server (MOSS) 2007. It will not be
free and requires X64 machines. SharePoint Designer 2010 will continue to
be free and will have both an X86 and X64 version. But as I mentioned it
can only be used to edit site in either SharePoint Foundations (SPF) 2010 or
SharePoint Server (SPS) 2010.