Microsoft Security Essentials is renamed to Defender on Windows 8.1.
Defender was a different program on XP, more a malware scanner than an antivirus.
On Windows 8.1, Defender is an antivirus, though you may want an additional malware scanner like MalwareBytes.
Just make sure you keep Defender updated on Windows 8.1.
I suggest using something like Classic Shell to get a better Start Menu.
http://www.classicshell.net/Then you can do a search for Defender to start it up and update it with new signatures.