This indicates an attack attempt to exploit a memory-corruption vulnerability
in Microsoft Excel.
This vulnerability is caused by an error when the affected software handles a crafted XLS file with a malformed object. It allows a remote attacker to execute arbitrary code.
Microsoft Office Excel 2000 Service Pack 3
Microsoft Office Excel 2002 Service Pack 3
Microsoft Office Excel 2003 Service Pack 3
Microsoft Office Excel 2007 Service Pack 1
Microsoft Office 2004 for Mac
Microsoft Office 2008 for Mac
Microsoft Office Excel Viewer 2003 Service Pack 3
Microsoft Office Excel Viewer
Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats Service Pack 1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's website: