Like every new Windows release, Windows 8 is more secure than the operating systems that came before it. That's due in large part to three major enhancements: An increased emphasis on UEFI Secure Boot optimizations, the extension of the SmartScreen Filter across the operating system, and the default inclusion of a more robust version of Windows Defender, which now protects against all kinds of malware--not just spyware.
Windows Defender's increased scope doesn't sit well with computer manufacturers, however. OEMs make beaucoup bucks by installing those trial versions of McAfee, Norton and other security suites you'll find bundled on boxed PCs. Windows Defender's default installation threatens that gravy train.
Microsoft tossed its partners a bone by allowing OEMs to deactivate Windows Defender in order to ship boxed PCs with alternative security solutions installed. That's all well and good from a "variety is the spice of life" perspective, but one side-effect that isn't so hot is what happens when you fail to register that third-party security software: Windows 8 doesn't automatically reactivate Windows Defender by default. In other words, your pretty new prepackaged PC is wide open and vulnerable to all the nasties of the 'Net.