I down loaded iso of windows 10 tech preview and installed it on a toshiba l505d-es5025 laptop. Ran it for a couple of days and then a vertical black box started to appear on the upper left of the screen. Only when I am at the desktop. Never while in an app or on web at first. Now appears at all places. I have a screen shot of this if anyone is interested to see. Is this a virus or Microsoft?