I've written here before about some reasons free, openly developed software generally
has fewer security issues than proprietary software. However, one would expect Microsoft to beat the odds, since they're capable of funding any development process they want. They can hire world-class programming and QA teams and make sure at least their software contains no bugs or vulnerabilities.
Of course we all know that doesn't happen, but it might one day. I'd like to point out that there's another fundamental reason Windows and Office, or any similarly proprietary OS and applications bundle, can't be as secure as a good Linux distribution. Since I used to be a
Gentoo Linux packager, I naturally consider package management to be the indispensable quality Windows lacks.