It's a little complicated, (my first post was referring to the "old" Windows Defender not being an antivirus), but the newly morphed "Windows Defender" is promoted by Microsoft to combine both antivirus and antispyware and it's automatically installed and running on Windows 8 UNLESS you install a third party security program. The new version of "Windows Defender" is supposed to use the same virus definitions as Security Essentials..
Unfortunately, I am always concerned about the effectiveness of any security product until it's reviewed and tested thoroughly.. When Windows Defender first was released on other versions of Windows, it wasn't the "best" antispyware program on the market and therefore, many steered clear of it.. Since Windows 8 now has a combo program of Security Essentials and antispyware called "Windows Defender", you might want to do your own checking as to the quality of the product.
Hope this helps.