The registry is still there, so the slowdown is inevitable. And with malware bypassing UAC 7 times out of ten, even using an antivirus infection will be hard to avoid for an "Average Jill" who gets easily tricked into getting infected.
True about the registry, and true about the inevitability, but I don't think too many people are getting tricked into infections. That is unless you mean limewire users.
I don't think the virus issue is going to be a huge problem for most users with Windows 7 and future Windows updates. Microsoft knows that the issue will forever be a stab at their OS and will (hopefully anyway) continue to prevent it from being a major issue. That along with users being a little smarter and email clients filtering on the server level, etc.
On a side note I think the bigger issue for Windows users is going to be it's openness and QC for 3rd party software makers and Microsoft. Running Avid on a PC was always an issue for me because of that very reason. Many of the "viruses" we had in our shop were caused by the mixing of 3rd party software, Avid, Boris FX, Microsoft, and HP.
With Apple, it's just Apple, Adobe, and . . . that's about it.