Yep, it's a big risk. (See reboot loop.) Nobody has attempted anything that big before....

Otherwise, I don't recall having a Windows Update problem recently -- certainly not this century -- and I haven't seen any malware either.

I'd be willing to bet that the malware problem will be significantly smaller with Microsoft doing the updating than it is at the moment, even though the majority of malware problems on Windows are down to human ignorance.

(I know I can run Windows 7 with no AV at all, and I'm certainly not the only one. I don't recommend it, but I have done it.)