I used to run Security Essentials. When I upgraded to 8, it automatically uninstalled it. It's baked into the OS now, along with a constantly updated blacklist of known malware programs if you try to manually install something bad. You can get around it easily, but they don't make it intuitive to do so for users who are not comfortable clicking around and exploring.
Above and beyond being comfortable not using an AV on Windows 8, I would go as far as to say that if you are using an AV on Windows 8, you're being taken for a ride by your vendor of choice. And I say this as an information security professional. I've tried to get a virus on Windows 8 without doing anything more than what it takes to get a virus on Windows 7. I did not succeed.
Above and beyond being comfortable not using an AV on Windows 8, I would go as far as to say that if you are using an AV on Windows 8, you're being taken for a ride by your vendor of choice. And I say this as an information security professional. I've tried to get a virus on Windows 8 without doing anything more than what it takes to get a virus on Windows 7. I did not succeed.