On the speed front, I'm talking specifically about MSE (that I don't notice it's affect) - I've had bad experiences with Norton grinding my PC down, horrible invasive product that one! Can't comment on Mac v PC speed impact.

I agree with you on the App Store front, think it will help to reduce viruses/malware (I think Windows 8 app store will be a huge win too), but it's unlikely that an app store will be the sole place to get software for some time to come. So until it's impossible to manually install software off the internet, I think we're going to need anti-malware/virus software.