Well the article's talking about having a way to safely install software - my general view is that while UAC and firewall-type mechanisms mean malware authors need to be more careful, they don't provide any firm security guarantees. So if an attacker can get you to run an executable they've provided, even as a non-admin user, they win. If you download executables over HTTP, and run them without taking steps to verify them, you are likely vulnerable; signature-based virus scanners are not to be relied upon, and any malware scan run after you run the program could at least in theory be defeated - if nothing else, by a Blue Pill-style hypervisor.

Now, how realistic a danger a HTTP MITM is it depends on your threat model; I don't think anyone's doing this on a large scale (other than governments in places like Iran and North Korea). But if you're worried about attackers targeting you specifically, then I think this is a valid threat vector.

MS tried to address this in earlier windows with digital signing of downloads (you get a warning if you try to run an unsigned executable, and a different warning identifying the publisher for a signed one, assuming the executable in question is marked as having been downloaded from the internet), and by integrating an app store into windows 8 (I think?). If done correctly, this would prevent this kind of attack - if you only ever run signed executables, and you trust the signers, you're safe. But the fact that even PuTTY, supposedly a piece of security software, is an unsigned download, suggests that this approach hasn't really spread through the windows software ecosystem yet. (The other approach that might work is extending UAC into some kind of full containerization approach, and isolating applications more fully from each other).