Right from the horse's mouth
The new Windows Vista will be a great success! After twenty years of hard work, Microsoft is finally going to create a secure, reliable operating system. Or at least, so they say:
Windows Me
"Windows Me: PC Health Features Keep PCs Stable, Secure and Reliable -- and Take the Frustration Out of Computing for Home Users" (source) [microsoft.com]
Windows 2000
"Our primary goal is to improve security and safety for all our customers -- consumers and businesses, regardless of size -- through a balance of technology innovation, guidance and industry leadership," Gates said. "We're committed to continued innovation that addresses the threats of today and anticipates those that will undoubtedly emerge in the future." (source) [microsoft.com]
Windows XP
"Windows XP is the most secure and dependable operating system we have ever produced." (source) [microsoft.com]
Windows Vista
"In Vista, it should be much more difficult for unauthorized programs (like Viruses and Trojans) to affect the core of the OS and secretly harm your system." (source) [extremetech.com]