Back in the XP days it was a veneer on GIANT AntiSpyware which as I recall was viewed as a separate component to anti-virus back then. As things evolved you saw combined firewall/anti-spyware/virus/adware from companies like Norton/Symantec and as you note MSE was really MS's anti-virus entry in Vista & 7.

Nowadays 'anti-virus' subsumes all those products into one unified thing.

https://en.wikipedia.org/wiki/Microsoft_Defender#Beta