A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
You have the Fixed Decimal Setting enabled. This feature is intended for circumstances in which you are entering a large number of values with decimal points. It automatically inserts the decimal point for you. Go to the File menu, choose Options then the Advanced page. There, scroll down to Editing Options and uncheck the "Automatically insert a decimal point" option. It is this setting that enables and disables the automatic insertion of a decimal point when you enter a number in a cell.
If this option is checked and in effect, you should see "Fixed Decimal" in th lower left hand side of the Excel application windows, just below the worksheet tabs.