Biggest problem with Windows: Registry system. That's what causes most of the issues and a large number of the exploits. Neither Mac or Linux have these issues.
Of course, MS will never get rid of the registry because it means not only would they have to completely retool the OS, all the software developers for Windows would have to retool their software, and NOTHING written prior to the new OS becoming available would be installable for the new OS, unless they also made an emulator for older versions of Windows (which they wouldn't release on the first round anyway, forcing people to upgrade... it's how MS works in general)