NetZero Software Does Not Install (Double Launch)

For my Internet connection, I have decided to use NetZero as my ISP.
When I tried to install the starting software for NetZero, the error shown below keeps popping up.
I have tried to double-check whether NetZero is, indeed, already running, but the same error shows up even when I launch the installation for the first time.
How do I go about repairing this? Thanks, everyone!

Problem Starting NetZero
The NetZero software is already running.
You must close the software before starting it.
Error Code: 02
