Don't under estimate the crafty determination of these folks. If it's connected to a network it's a target, and someone's going to figure out how to attack it. The code behind a lot of viruses, and etc... is excellent and tight and sometimes, down right deviously ingenious. If all those people were after Apples and Linux systems instead of Windows systems...they would get through.
There's too many gullible and stupid users out there. The intelligence curve for Linux is still pretty high. Businesses, with all their average stupid users, haven't switched over to Linux yet. As business starts to accept Linux more and more (as it should) the Stupid User Curve will shoot up....patches won't be as diligently applied...Email programs will expand in Automation Functionality...the door will open. It's inevitable.
If there's a Stupid User at the keyboard and it's connected to a network, there is no such thing as a secure computer.
There is also no such thing as an entirely safe automobile. B ut I'd be safer in a Mercedes than in a Yugo. Accumulated data over numerous years have proved this point.
Likewise, some companies' operating systems have regularly been shown to have security problems. Other operating systems have managed to be proven secure in it's default configuration for over five years.
Any operating system or software is vulnerable to a stupid user. However, some operating systems and software packages have proven to be more vulnerable than others, even given the same stupid users. That's due to the design philosophies.
As long as Microsoft keeps treating security as a PR problem instead of a design problem, malware, viruses and debilitating bugs will continue to plague the users of their software.