An Excel window opened when I previewed an Excel file in Window explorer.

Hello binodsharma​,

that is normal Windows-Office behavior.

Explorer's preview pane uses Excel's preview handler, which starts a background Excel COM instance and, on some builds, can briefly show a blank Excel window when you select an .xlsx file. 

If you want to avoid it, you can turn off the Preview pane in Explorer, View > Preview pane, or disable preview handlers from File Explorer Options > View by unchecking "Show preview handlers in preview pane."

If this only started recently, make sure Office and Windows are up to date and, if needed, run an Office Quick Repair from Control Panel > Programs > Microsoft 365 > Change.

Let us know if it works.