Hi, I'm David an independent advisor and user just like you. I'll try and help you solve this problem.
It looks like the box is a Google Chrome window as Chrome is showing it is open (the icon at the bottom of the screen has a line under it). Try right-clicking that Chrome icon and choose "Close window" - does this make the box disappear?
If it comes back again the next time you restart there could be something in your start-up list which is trying to launch Chrome. To check that press CTRL + ALT + DELETE on the keyboard, from the options which appear choose "Task Manager", from there click "More Details" in the bottom left of that window and then the "Start-up". You can selectively disable/enable startup items from there.. you may need to experiment to find the item which is causing the pop by disabling an item and then restarting to check.
Regards,
David