I'm not convinced that viruses and other security problems are more common with Windows OS just because it is more popular, though clearly it is a major factor. Microsoft push a lot of integration technologies (for want of a better word/expression) like ActiveX, Internet Explorer components within other apps, dynamic scripting across applications etc. More generally, the tight integration of the key Internet apps (email, browser) with the desktop operating system seems to be the primary course of a lot of problems these days. No other OS does this as much as Windows and perhaps this is one reason it has more problems.