What I've found with Norton Internet Security and the PCs I've had to repair for my customers (and yes, the latest versions) is that more often than not, it's ineffective. The machine gets infected with some kind of malware and the first thing it does is partially disable Norton. Then, you've got a real problem, because now Norton has co-opted the TCP/IP stack with its craptastic firewall. So, you can't pull down any tools over the network, and you're stuck with using a USB key.
What's worse is that more often than not, you can't uninstall it because the installer isn't smart enough to "heal" itself if conditions aren't EXACTLY what it's expecting. So, now you've got to download a special uninstaller from Symantec and hope and pray that it will actually rid the machine of Norton - and that's before you can even start nuking the bug that started the whole mess.
I'm sorry, I've spent more time fighting Symantec products (including their Enterprise, non-AV products) than I have using them, and avoid them like the plague they are on my personal machines.
On Windows, I use MSE and have not had a single malady on any of my boxes or VMs. On my Mac, I don't use an AV product (which by the way, will not protect you from yourself - you still need to practice safe surfing).