It seems that there are some rather nasty updates to either Windows itself or Java running under Windows that is removing the file association for .jar files for a number of users. It is not clear at this time why this is happening, but it is preventing people from running HD by double-clicking on the HD6.jar file (or any shortcut they've created).
I'll use this thread for troubleshooting this issue with folks -- the fix information in this post will be edited as needed to help out.
The following is the most direct way to check and/or set the file association for .jar files under Windows:
1. Open a command prompt (Start -> Run... -> cmd)
2. To check the file association for jar files, enter the following (please post the output of this command in this thread if you are still having problems):
ftype | find "jarfile"
3. To reset the file association for jar files, you would enter the following, replacing the path to javaw.exe with the absolute path to the file on your system :
ftype jarfile="C:\Program Files\Java\jre7\bin\javaw.exe" -jar "%1" %*