You can use it without registering FOR TEN DAYS.
After that, you must delete a small .DAT file contained here: (Windows)
C:Users/your username here/App/Data/Local/Fluenz
THAT'S ALL. You can then go ahead and use for ten days, and repeat the process to reset 10 day period without registering the software.
Delete this file-
For the 5 parts (5 ISOs)