Hi,
Thanks for your post in Microsoft Community.
I understand that you are having issues with the taskbar disappearing by pressing Alt+tab while running the game in windowed mode.
Have you installed some third-party security management software?
If so, you can try to uninstall it first, and then try the following:
Press Win+R, enter and run msconfig, in the Services tab, hide all Microsoft services, and disable other services.
Press Ctrl+Shift+Esc to open the Task Manager, go to the Startup tab, disable all startup items.
Then restart the computer.
Disclaimer: A “clean boot” starts Windows with a minimal set of drivers and startup programs. It helps to determine whether a background program is interfering with your game or program and to isolate the cause of a problem.
These steps of "clean boot" might look complicated at first glance. However, to avoid any trouble for you, please follow them in order and step-by step so that it will help you get you back on track.
After rebooting, run the command prompt as administrator, enter and run the following command:
Dism /Online /Cleanup-Image /ScanHealth
Dism /Online /Cleanup-Image /CheckHealth
DISM /Online /Cleanup-image /RestoreHealth
sfc /scannow
Disclaimer: Running Microsoft Safety Scanner can help you check if the system is disturbed or malicious software exists. To avoid any trouble for you, please follow them step-by step and back up all your personal files first to ensure you do not lose data.
Then restart the computer again to see if that resolves the issue.
Please feel free to let me know how it goes.
Kirk | Microsoft Community Support Specialist