Defragmentation is something from prehistoric ages of Windows (if I remember correctly, they made it automatic in Vista). Malware, adware etc are the same class of problems as unprotected sex: if you are careful, you are safe and will never see it. It's not a Windows "feature".
I've always felt this was just the law of large numbers: The world has more PC users running Windows, so people who are writing malware would be casting the widest net by attacking PCs running Windows and since the bad guy is also likely using/familiar with a PC running Windows.
The second the paradigm changes, the value equation changes for everyone involved.
