Started a W8 machine in a hurry to do something. Huge banner over everything saying free upgrade to 8.1. One button saying "download from App Store". No Cancel, No or don't option.
Didn't want that, so tried to do something, anything else. Nothing works.
Got really, really, really annoyed.
Then thought a bit, hit the button, then opened Task Manager and killed the App Store.
It now seems OK. The banner hasn't appeared on the second machine here.
Maybe this posting will save some small percentage stress somewhere.