The coordinate system definition for these layers does not match the ESRI .prj extension or format for WGS 1984. Therefore, these SDC layers cannot be projected to a different coordinate system (datum).
The coordinate system for the following SDC layers in the data_update_2008\usa\census folder is 'Lat Long WGS84', instead of 'GCS_WGS_1984'.
The coordinate system for these layers is defined as
GEOGCS["Lat Long WGS84",
SPHEROID["World Geodetic System of 1984",6378137,298.257222932867]],
Since the coordinate system definition for these layers does not match the ESRI PRJ name or format for WGS 1984, these layers cannot be projected to a new coordinate system that uses a different geographic coordinate system (datum).
The coordinate system definition should read:
The well-known text (WKT) string has been formatted to make it easier to read.
Using ArcCatalog, copy any required layers to a local hard drive.
In Windows Explorer, change permissions on the files or folder that contains the data to allow read-write access.
Use the Define Projection tool in ArcToolbox to define the projection as WGS 1984.prj. This projection file is located in Coordinate Systems > Geographic Coordinate Systems > World.