Avast works for me.
I agree, I have found Norton caused my PC to grind to a halt. McAfee was better but that too became much too heavy. So I tried Kaspersky - great in 2009, but 2010 - oh er! So in 2011 I looked at AVG - but that didn't work for me. So then tired Avast free version. Excellent.
I find that too many bells and whistles in some Security software are unnecessary. So now only run AV and Firewall.
My ISP - Sky (in the UK) - seems to be top notch at keeping Spam away from emails etc. Plus I don't visit "dodgy" web sites and NEVER open unexpected attachments in e-mails.
Keep it simple and Avast does this for me!