This indicates an attempt to exploit a remote code execution vulnerability in Microsoft Excel.
The vulnerability is a result of the software's failure or correctly handle conditional formatting values. It can lead to an exploitable stack overrun when processing conditional formatting BIFF records from a worksheet in the Workbook stream.
Microsoft Office Excel 2000 Service Pack 3
Microsoft Office Excel 2002 Service Pack 3
Microsoft Office 2004 for Mac
Microsoft Office 2008 for Mac
System Compromise: remote attackers can execute code on vulnerable systems.
Apply patch, available from the web site.