I loved Norton, then I hated Norton, and for now, I love it again.
At one time it gobbled up all my computing resources, and my computer became slow as mollasses. Now, with the newer versions, and newer operating systems, it's back on track. Just be sure to program if for your full system scan in the wee hours of the morning (2-4AM) and leave your computer up all night, but log off of all programs.
I let the computer do a full scan once a week.
No problems with anything but new spam from arabic authors. No idea how I got on their list, but I now get about 6-10 spams a day in my spam file written in arabic.
But no, I don't have any problems like what Don is seeing with his double links. These things are usually blocked at the Norton firewall before they even get access to my computer.
Finally, if you system is slowing down for any reason, it's probably the memory. Always get twice the rated memory minimum for your computer, or you'll definitely be needing a memory upgrade later. Better to do it upon initial purchase.
Memory is far more important than CPU speed in Microsoft based systems. They are based on virtual windowing systems where each window is essentially a second computer, and each computer requires it's own memory, so to speak. You cannot have to much memory.