To suggest that the *main* reason that everybody finds problems with MS is its prevalence completely overlooks simple facts. For example, for a very long time, lots of security holes were left open by default rather than having to be opened by the user (displaying emails as html, displaying external images in there & executing scripts too, a variety of scripting turned on by default & just to be obscure, usermode access to I/O ports only closed after Win98). This theme is still true now although to a lesser extent with SP2.
I don't think that MS have taken security seriously enough & have sacrificed it for profits.