I groaned this morning when I opened the letter from BT about this. I have avoided Norton antivirus products for years because every PC I've ever used them on has subsequently had problems. I first bought it in the mid 1990s when I had my first Windoze based desktop and later stopped doing so because it caused a massive slowdown of everything; only for normal service to be restored after uninstalling the whole thing.
Let's hope they've improved the product because I have just installed it...