Copy from above cell

[fusion]

Hi...There is a cell in a particular workbook where the data will be the same in each row for that cell but the number of rows can vary each time. Rather than do a copy and paste I'd to know if there is a macro or function available that when a user clicks or move the cursor in that particular cell the data from the above cell is automatically copied into the cell. The data in the cell is a code "AAB" so if it is easier this can be hard coded.
I would like the macro/funtion to automatically start when the workbook is opened.
I appreciate some help with this please.
Thanks.

[mikerickson]

This is a cross-post from http://www.excelforum.com/excel-programming/780820-copy-from-above-cell.html

Since there are more responses there, the discussion can continue there.