Windows Defender, the malware and spyware utility first included in Windows Vista, got a major upgrade with Windows 8: virus protection. Best of all, Windows Defender is included in every edition of Windows 8 (including RT) so every Windows 8 box will have malware, spyware and virus protection automatically.
Those of you that have used Microsoft Security Essentials on Windows 7 will find the interface looks very familiar if not identical.
I have always liked the performance and reliability of Microsoft Security Essentials compared to third-party anti-virus utilities. So far performance and reliability appear to be the same with its twin, Windows Defender on Windows 8.
If you attempt to install Microsoft Security Essentials on Windows 8 the installer will politely tell you your computer is already protected.
So save yourself some money and stick with the default protection, it is actually pretty good.