This from the HELP file in Excel (on Circular reference )
Locate and remove a circular reference
If the Circular Reference toolbar is not displayed, click Customize on the Tools menu, click the Toolbars tab, and then select the Circular Reference check box.
On the Circular Reference toolbar, click the first cell in the Navigate Circular Reference box.
Review the formula in the cell. If you cannot determine whether the cell is the cause of the circular reference, click the next cell in the Navigate Circular Reference box.
Note The status bar displays the word "Circular," followed by a reference to one of the cells contained in the circular reference. If the word "Circular" appears without a cell reference, the active worksheet does not contain the circular reference.
Continue to review and correct the circular reference until the status bar no longer displays the word "Circular."
When the Circular Reference toolbar appears, tracer arrows appear that point out the cells that depend on the formula.
You can move between cells in a circular reference by double-clicking the tracer arrows.