Avoid Norton, because it's Norton. Same as McAfee.
Best free one by a country mile is the recently released
Microsoft Security Essentials.
Up until a few days ago, I'd have said avoid NOD32 on "fast computers" until they've sorted out the bug which stops you saving things in certain folders, but it seems they've just released an update (4.0.474.0) which fixes it.
The other best commercial AV is Kaspersky, which you can
get for free if you bank online with Barclays.