If you use Windows XP or below you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Norton Safe Web program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Norton Safe Web application and click on Uninstall.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys in your system and download and install a new copy of the software.