It's advised to install this application after you did a reboot of your computer. This way you get sure that there aren't any programs running. However, after you rebooted the PC, try opening Task Manager by pressing the Ctrl + Shift + Esc keys at the same time. Search through the list of running programs and close all browsers or messaging applications like Skype, YMessenger, Firefox etc. After you did this, try installing the Nurago Web Meter application. You shouldn't encounter any errors.
