If you have previously used the right click and opened with \path\to\your\javaw.exe then you will need to remove the following registry key.

[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.jar]

Then run

C:\>assoc .jar=jarfile
C:\>ftype jarfile="C:\path\to\your\javaw.exe" -jar "%1" %*