Windows 8 (and Windows RT) integrates a more robust version of Windows Defender
(and uses that name) for its anti-virus and anti-malware protection. Windows Defender in Windows 8 provides the same level of protection
against malware as Microsoft Security Essentials (MSE) on previous operation systems. Since the new Windows Defender is integrated you cannot use MSE with Windows 8...in fact MSE is blocked from installation.
Windows 8 SecurityWindows Defender on Windows 8 - Introduction and FAQs
Microsoft is adding a bunch of significant new security features in Windows 8, including Early Launch Anti Malware (ELAM), Unified Extensible Firmware Interface (UEFI), and the first edition of Windows Defender to protect not just against malware but also against viruses...Until Windows 8, Windows Defender protected against spyware only. Now, it also takes on viruses and other breeds of malware..
If you want to use another anti-virus you must disable Windows 8 Defender before installing a different antivirus software
Although McAfee is as good as any other well known anti-virus program, it requires numerous services and running processes that consume a lot of system resources and often results in complaints of high CPU usage. Anti-virus software components insert themselves deep into the operating systems core and create files/folders/registry entries in various locations. If you do a Google Search
you will find there have been numerous complaints about it affecting system performance. Those issues plus the cost factor are the primary reason many folks look for a free alternative as a replacement. McAfee is better utilized in an Enterprise system environment protecting many client computers.
I would stick with Windows Defender in Windows 8. Other alternatives are listed in Choosing an Anti-Virus Program