Below are steps you can take to ensure your sheets and workbooks open in the existing window:

1. Click the ‘View’ tab on the ribbon, then expand the Window group.

2. Click the ‘Arrange All’ button and select ‘Tiled.’

3. The sheets will tile to display in the same window. If you have multiple workbooks open, repeat this step for each one.

4. Click the 'View' tab again, then click the 'Windows Options' drop down menu in the Window group.

5. Make sure the 'Windows in Taskbar' box option is checked.