Well the best thing would be to stop users using them - in that sense they'll never get infections
But if I put my rogue hat on for a minute, I can see downsides to the OS automagically having a built in antivirus.
It will lead to patches and upgrades merely for the exploits of having the thing itself installed.
They should work harder on other things tho. Free AVG is, as it suggests, free - which means everyone can have it.