Do I need to install antivirus on Windows 8?
Think of your computer like your home with windows and doors as entry points. The Windows Defender (or Microsoft Security Essentials for Windows 7 and previous versions) that comes with Windows 8 by default would be like basic door and window locks and very basic alarm system; basic protections, but not good enough for good protection. Avast free or many of the other free or paid antivirus software (other than McAfee and other low rated antivirus software) offer much more protection, detection, and cleanup capabilities. This would be like deadbolts, bars around the windows, and surveillance of almost everything.
What does this mean? No, you don't need a different antivirus program, but why leave it to chance? Also, it is recommended to install Malwarebytes AntiMalware Free and scan at least once a month just for precaution, or install SUPERAntiSpyware (click on the red "Free Edition Download" button near the top of the page to download) and scan at least once a month for precaution. This is like scheduled inspection and maintenance.
I wrote the following article a while back outlining the relationship between using admin accounts in windows and viruses. http://diaryofageek.com/administrator-accounts-the-root-of-windows-os-virus-woes/
Windows 8 does have a basic antivirus solution built-in ("Windows Defender"), but as Rajaa and Hovsep say, it's not very good.
I disagree with Hovsep on the "safe computing" angle. In my opinion, practicing safe computing includes using a competent AV package, since you can encounter malware anywhere - on the net, on a USB drive etc, and it's not really related to how you use your computer (unless you don't connecto your PC to anthing!).
I can recommend Avast as a free, excellent and relatively "light weight" package, but there are many other excellent free options out there. Choose one of them and you'll be a lot safer.
When Windows release Microsoft Security Essentials, it was really faring good is all benchmarks for free anti-virus solutions. However, off late Microsoft security solutions has slipped and actually is faring at the bottom on benchmarks. I have been using Windows 8 since it's launch and only had the Microftsoft security built-in in Windows 8 working. But just this monday I read about this article and now have AVG Free Antivirus 2014 installed on Windows 8 along with MalwareBytes and kept the Windows Firewall. Incidentally, AVG got rated best in the free software category in the article.
well if you practice safe computing and internet browsing then Windows defender should be enough but it is always recommended to get a better antimalware security tool.
Included in Windows 8, Windows RT, Windows 8.1, and Windows RT 8.1, Windows Defender helps guard your PC against viruses and other malware. For a PC running an older version of Windows, you can download Microsoft Security Essentials.
How do I find and remove a virus?
Understanding Microsoft Anti-Malware Software