This indicates an attack attempt against a remote Code Execution vulnerability in Microsoft Office Excel.
The vulnerability is caused by an error when the vulnerable software handles a specially crafted ".XLS file". It allows a remote attacker to execute arbitrary code.
Microsoft Office XP Service Pack 3
Microsoft Office 2003 Service Pack 3
Microsoft Office 2007 Service Pack 2
Microsoft Office 2010 (32-bit editions)
Microsoft Office 2010 (64-bit editions)
Microsoft Office 2004 for Mac
Microsoft Office 2008 for Mac
Microsoft Office for Mac 2011
Open XML File Format Converter for Mac
Microsoft Excel Viewer Service Pack 2
Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats Service Pack 2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's web site: