The folder C:\Documents and Settings\User\Application Data\J River\Generic\Temp
Nothing should be using that as the temporary folder (using Generic means something failed to load product information). I wonder if there was a bug with a certain FLAC plugin?
It is safe to delete the files.
The temporary folder should be:
C:\Documents and Settings\User\Application Data\J River\Media Center 13\Temp
And it should be deleted on shutdown.
Let us know if any files come back after a delete.
Thanks.