Ok, so I think it is clear that any anti-virus software is going to slow down your computer. The control you have is in what way you are ground to a near halt! Both Norton and McAfee are easy to use, so no difference there, and both seem to have kept me virus free!
I have been using Norton for about 3 years, and was sick of how slow it made everything, and McAfee was cheaper so I took the plunge. Props to Amazon, I ordered it at lunchtime on Monday and it was with me Tuesday morning!!
I didn't install all the bits that previous reviewers told me not to, which will have helped my experience from the start, but I do prefer it to Norton. Should also be said though, my computer started up in about 1/10 of the time during the transition when I had no anti-virus software installed! And, true to form, Norton is a pain to uninstall!
On startup, Norton stops you from being able to use your computer at all until it has finished faffing around, which is a pretty long time! McAfee faffs for longer, but you can use your computer at a slightly slower speed for this time.
I like the way McAfee doesn't virus scan a Word/Excel document every time I try to open it, this have literally saved me about 30 minutes a day in waiting over Norton!!
Also, McAfee does seem to let you tell it what to do more. I told my mother's computer (which has Norton) to ask her before it runs Windows Update (she's on dial up and it's a pain), but Norton was having none of this, and forced me to change the settings back to automatic update. I was displeased... McAfee, however, has a "disregard this problem" button, so it carries on running happily when you change something. I like this freedom!
I think it totally depends what you want from anti-virus software, but McAfee is definately the one for me! Now if I could only work out how to tell it to virus scan once a month not once a day...