Excel has an automatted method:
The concept is the same as what lenze provided, but it shows you what it was changed from. This will not allow macros.
Highlight changes as you work
When you mark changes in a workbook by using the Highlight Changes command, you also turn on workbook sharing (if the workbook is not currently shared) and the change history. For information about changing the change history settings, click .
Some Microsoft Excel commands and features are not available in a shared workbook. For more information, click .
On the Tools menu, point to Track Changes, and then click Highlight Changes.
Show Me
Make sure the Track changes while editing check box is selected. This check box turns on workbook sharing and the change history.
Select the When check box, click Not yet reviewed in the When box, and then click OK.
If prompted, save the workbook.
Make the changes you want on the worksheet. Microsoft Excel marks cells that you change, insert, or delete with a highlight color.
Note When highlighting changes, Microsoft Excel does not mark some changes, such as formatting. For more information about which changes are marked and ignored, click .