I wouldn't piss on Norton to put out a fire.
Yes, it will slow ANY machine down. Not just the old stuff. Norton is a hog, Vista is a hog, and hogs slow things down. I've got an Athlon XP over here that will smoke everything on the shelf today. Why? Because it runs light, that's why. Take the biggest quad-core you want, with the fastest RAM you can get, and the biggest internal caches you can find, and install Norton. See what happens.
You have just ruined your machine. That is a cold, hard, fact.
I do a lot of go-fast work on computers. It is cheaper than cars, what do you want?
I'll take any machine and squeeze so much life out of it you wouldn't believe it. Of course, building is more fun, but nothing rewards like taking a hog of a machine, like a Ferrari towing a load of bricks around, and cutting those bricks loose so the thing can run like it is supposed to.
Removing Norton is without a doubt, the biggest single software effect that can be had on a machine, unless you count going from Vista to XP on a machine that can get away with it.
Those guys used to have some great programs. Norton Commander was so good, I've got a copy on a shelf, still in it's shrink wrap from the days of DOS.
Now, however, their programs are bloated, and like some sort of tentacled fiend, they infect even the darkest corners of your computer with their overloaded code. They are so infectious, in fact, that they are nearly impossible to remove in their entirety.
Norton has been known to slow things down to such a vast degree, that performance in intensive programs can be cut in half as a result. Absolutely without merit of any kind.
I currently run a program called Avast. I got it because it was the smallest program available. AVG has become bloated over the years as well, and I was no longer satisfied with it's performance. Avast is so small, and uses so little memory with it's active processes, that I have not noticed any performance difference at all from running it.
On top of that, I have used Avast to help counter and destroy the newest dreaded Antivirus advertisement trojans that are being passed around. Avast stops them dead in their tracks, and if installed on an infected machine, will destroy most of the issues all by itself. A program called Malwarebytes destroys the rest of the infection entirely.
Without Avast and this other program, such an infection requires wiping your hard drive and starting over. I've never seen another program catch this stuff so quickly, and do it without causing performance issues.
Norton is rubbish.
If you must have it, by the powers, do the upgrade. Don't go and try and install a new version. There is so much crap going on in that program that you do NOT want to rock the boat, believe me.