Should you get a pop-up like this one. Do not follow the advice in the window, and in particular, do not call the number in the window. Just close the browser immediately. DO NOT click on any part of the window, as this can trigger a virus installation. If the pop-up is preventing you from closing the browser, then reboot your computer. Also, make a mental note of the site you were visiting when the alert popped up. Very often it will be a news site or a content aggregation site; the type of site with lots of ads in the borders or strewn throughout the articles. Avoid the site if possible, and certainly avoid clicking on anything “interesting” in the margins.
Microsoft will never request that you call them about a security issue (in fact I can’t think of any instance in which Microsoft will request that you call them). Calling the number will lead you to an offshore site that will tell you that they will clean up your system for a fee, and once they have remote access it’s anybody’s guess what will actually happen.
Don’t do it!