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 .XLW file. It allows a remote attacker to execute arbitrary code.
Microsoft Excel 2002 Service Pack 3
Microsoft Excel 2003 Service Pack 3
Microsoft Office 2004 for Mac
Microsoft Office 2008 for Mac
Open XML File Format Converter for Mac
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the patch, available from the vendor's web site: