Windows Defender is a feature of Windows 8 and is enabled by default in Windows 8 Release Preview however you can also use one of those shown below (click on the image for details), if you do Windows Defender will turn itself off…
“Some other companies might offer products that appear to be antimalware apps, but when you install the app, you might also be installing viruses or malware on your PC without knowing it. You should carefully investigate the source of any apps, including antimalware apps, before downloading and installing them.
Important: Before you install an antimalware app, check to make sure you don't already have one enabled on your PC. If you do, be sure to remove the app you don't want before you install the new one. It can cause problems on your PC to have two different antimalware apps enabled at the same time. As noted earlier, Windows Defender will turn itself off if you choose another antimalware app, so no further action is needed in this case.”