I have Norton Security with Backup running on an older machine with XP SP3***. I've noticed since the install, that I'm having regular performance issues, bu mostly related to Firefox. Pages load really slow, and there are times that Firefox get stuck in "Connecting" mode (grey circle just spins and spins...) If the load never completes, the only way out is to re-boot the PC. I'm not sure if it is related to NSBU or not, but I did notice the slow-down and non-loading issue following the install. Firefox is up-to-date.
***I know, I know, I know...I need to update and or upgrade my hardware, but $$$...