The md5 hash file available via the GWX d/l thread is not correct - it appears to have been corrupted when u/l (ironic really, as the source is correct).
Get the correct one here...
http://hosted.filefront.com/JScones/1890008
To use, d/l and save to the same folder as the GWX pieces. Then open MD5Compare, click on "Compare from MD5 file" and press "1. Select MD5 file...".
Browse to the GWX.md5 file and select it.
All going well you should get 6 ticks. Any crosses against any of the GWX files means corruption.