I would add some info about Windows Defender in W8 --
In Windows 8, Microsoft decided to include their own anti-virus software built in called Windows Defender. This software is the equivalent of Microsoft Security Essentials for Windows 7, Vista and XP but is pre-installed by Microsoft. However, it is
not the same as Windows Defender in Vista, since that did not offer real-time malware protection, despite the same name. Some more information can be found here -->
Understanding Microsoft Anti-Malware Software 2012 ~ Security Garden <-- for those who are interested