There is an Option setting for this, but getting to it varies between versions of Excel. The easiest way to stop it without knowing your Excel version is like this.
1) With any worksheet active, press ALT+F11 to go into the VB editor.
2) Look for the Immediate Window... if you don't see it, press CTRL+G to make it appear
3) Type this into the Immediate Window and then press the Enter Key...
Application.FixedDecimal = False
That should stop the problem you are having.