Why a virus wouldn't work against this Ai. (Spoilers)
At the end of the movie there is a virus that essentially kills the Ai and this kills the power grid also because the Ai is in everything.
This is good. Eventually kill the Ai and even the power goes. Forget about the death toll this would have on the world. Except with this Ai there is one flaw.
The Nanobots. We see throughout the movie their ability to repair humans, plants and especially the Solar Pannels.
If they have the ability to repair anything. Including the solar panels then they also have the ability to be solar panel powered themselves! Assuming they are operating in a collective state over some sort of advanced WiFi. The moment a virus tried to destroy the Ai, over the span of trillions of these nanobots the Ai could transfer itself. Rebuild in a world without a human interface available and essentially recreate the entire world as it sees fit.
In fact. At the very end. This may be the very thing that the droplet of water from the sunflower was saying.
I'll be honest I haven't really thought too much of this. But an Ai will figure out many different fail safes and virus protection and survival will be at the top of its list.