Here are a few different things that could be causing the problem. The most likely is that Excel is configured so it automatically places a decimal point in your entry. You can tell if this is the case by looking at the status bar; if it has the words “Fixed Decimal” appear there, then this is the cause.
Turn it off by following these steps if you are using Excel 2007:
Click the Office button and then click Excel Options. Excel displays the Excel Options dialog box.
At the left of the dialog box click Advanced.
Under Editing Options, make sure that the check box for “Automatically Insert a Decimal Point” is cleared. (See Figure 1.)