I'm implying that the only reason that I could think of to change users was to prevent access to certain programs and files. A-La school systems. Personally, whenever someone uses my computer, I make sure that they never change any of my settings, such as background, screensaver, etc. But then again, I don't share my computer with my family very often, and I doubt I could stand doing so. Meh.
I didn't shut of windows defender when installing, and it works great, but a couple of people's problems have been solved by that.
As for keeping it on topic, the topic was "Return the product before 30 days or you can't". I agree with you in that regard... If you try to claim the warrenty on a dishwasher one day after the warrenty is up, you generally won't get help with it. (Unless you bought it from somewhere that is willing to waste a lot of money on customer support). In your case, it would have been twice the duration, so more like 3 years after the warrenty is up.
Give the windows defender shut off a try.
afterthought: all of your issues sound like they're stemming from one cause