If you can't catch it with Task Manager then look at every line in AUTORUNS. That's a Microsoft tool to list all items that run on a PC. Link follows but you have your work cut out for you. Google is your friend on each item.
So a white box just appears in top left corner of the screen, it's on top of everything and then it's disappears after a minute or so, here's a screenshot: https://i.imgur.com/UHOerOG.jpg please help, thank you.