A new version of the native WinGrass installer (note, not OSGeo4W or Cygwin) has been posted on the GRASS download page.
This release is based on GRASS 6.4.0svn (r38537) and fixes some problems with the Location Wizard among other things.
Download here: https://grass.osgeo.org/grass64/binary/mswindows/native/WinGRASS-6.4.0SVN-r38537-1-Setup.exe
For known issues check:
https://grass.osgeo.org/grass64/binary/mswindows/native/#Known_Issues
Please test and report any problems. Thanks to Colin Nielsen for packaging.