If you don't get an audible beep when the phantom is plugged to the PC and powered up, the driver isn't working.
Download a program called CCleaner (Crap Cleaner), remove the driver, use CCleaner to scrub the artifacts from the registry and try again on the driver install.
If you do get the beep, but there is no communication, you can uninstall assistant, scrub registry with CCleaner and reinstall.
The phantom has to be powered up when the cable from it is connected to the computer. The driver can recognize it if the power is applied after connecting the USB cable, but there has to be power (just power - no motor start) on the Phantom before the driver will see it.
If that does not work, I would try another computer or turn to your dealer for help.
There is a seldom referenced "reset" for the NAZA that involves removing the top of the shell and using a servo cable from F1 to F2 and power cycle the battery. I don't know if it would do any good, and would seek help from someone who has used that procedure with success before attempting.
I am not familiar with Win 8, but those procedures are good up to Win 7.