That said, there are tools an OS can do to mitigate the damage that viruses can do. So while there will always be a smarter virus, there can always be a way to only let the smart virus get to something that it could have gotten to no matter what.
Its like Google Chrome. Google Chrome itself is not perfect, has MANY (those we know of, those we don't) security flaws, BUT getting out of the sandbox that Google Chrome provides is a very difficult problem. The basic idea is really to let viruses happen. Just let them get made. There will always be a security hole to exploit. As long as the damage the virus can do is very limited and moot it won't matter.
That might be what google is doing. Maybe they are just making a super-glorified browser-based system in which google chrome's sandbox protects the user while googles native client allows all sort of cool programs to run and in the end you are still on windows, but the security risk is absolutely minimal.