This isn't needed on Windows 8, Defender now replaces it. (Defender is now a full antivirus/antimalware suite type of thing).
After you uninstall any Norton or McAfee you might have, you should get a prompt that your computer is not protected and will ask to turn on Defender. If not, you can search for Defender on the pop out charm menu, then click (might be right click) and it will open the Defender program where you can turn it on.
Windows Defender for Windows 8 and Windows RT provides the same level of protection against malware as Microsoft Security Essentials. You can't use Microsoft Security Essentials with Windows 8, but you don't need to Windows Defender is already included and ready to go. But if youre looking to protect a PC with an older version of Windows, you can use Microsoft Security Essentials to help guard against viruses, spyware, and other malicious software. It provides free* real-time protection for your home or small business PCs.
http://windows.microsoft.com/en-us/windows/security-essentials-download