If you enter the security message into Google there are quite a number of results for this problem which seems to be primarily/exclusively an Internet Explorer 8 one, is that what you're using? There are instructions on these pages for how to turn it off, it's in the security settings for the browser. There is, I think, also a way to say which site is a trusted one and thus stop the warning for individual sites but keep it for others where needed.
I don't use IE so can't help much more than that but I do get a warning from the browser I do use, Firefox, when accessing ATP from my Windows 7 laptop. What it says is "Firefox prevented this page from automatically redirecting to another page", then there is a button to click for "Allow". I don't bother clicking "Allow" because the page loads normally regardless. It's not a new thing, I was getting that before the site changes so it likely isn't relevant to Ginger's problem.
