Hi. I'm David, and I'm happy to help you.
This large black square is usually a corrupted or frozen window frame from an application that opened automatically when Windows started. I suggest using the Task Manager to safely diagnose and isolate the exact program causing it.
Please try these steps:
- Open the Task Manager by pressing "Ctrl + Shift + Esc" on your keyboard;
- Make sure you are on the "Processes" tab and look at the top section labeled "Apps";
- Right-click on the listed applications one by one and select "End task". The moment the black square disappears, you have found the culprit.
To prevent this from happening every time the computer turns on:
- Click on the "Startup apps" tab on the left sidebar of the Task Manager (it looks like a small speedometer icon);
- Locate the application that caused the issue, right-click it, and select "Disable".
If you are unsure about any of the applications in the list or have doubts during the process, please share a screenshot of your Task Manager, and I can help you identify the program. Use the shortcut "Windows + Shift + S" to use the screenshot tool.
Feel free to ask any questions you may have.