Windows XP has known security issues; despite its continuing popularity, it is exploited more often and more easily than newer versions of Windows. Windows Vista added a number of protective mechanisms that made it more secure, but at a price: the “in your face” aspect of User Account Control annoyed users and arguably was one factor that led to widespread resistance to upgrading by businesses and individuals. Windows 7 toned down the visibility of its security while continuing to incorporate new security features that worked more transparently. The word is out that Windows 8 is expected to follow that same trend. You can read more here:
http://www.infoworld.com/t/windows-security/windows-8-security-stronger-gentler-174404