Grey rectangle on top left of the screen

It is probably supposed to be a "hidden" window from a shoddily written application. To figure out which process it belongs to, download Process Explorer, an excellent diagnostic tool from Sysinternals.

Run the program and accept the EULA, no installation required. In the toolbar there is an icon that looks like a target, titled "Find Window's Process." Click and drag from that icon to the gray region. When you release the mouse, Process Explorer will highlight the process that owns the window.

The Description and Company Name columns might help identify the responsible application, or you could just search the Internet for the process name. Double-clicking the row produces additional interesting information.