I use Linux not windows unless I'm fixing someone's computer but I believe windows 8 comes with essentials which is an antivirus. also believe lava soft ad aware and malwarebytes which are starting to include antivirus work in windows 8 as well.
Avast Free Edition is perhaps the free, most reliable and light weight antivirus which has now been also released for Windows 8. But, Windows 8 ships with Widows Defender installed in it. You do not need an antivirus unless until you want much security. I'm also a Windows 8 user and till now has not yet found any reason to install an Antivirus.