This indicates an attack attempt against a buffer-overflow vulnerability in Microsoft Office.
The vulnerability is caused by an error when the "MSO.DLL" library handles a malicious ".xls" file. It may allow remote attackers to execute arbitrary code by sending a crafted XLS file.
Microsoft Office XP Service Pack 3
Microsoft Office for Mac
Microsoft Office 2004 for Mac
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's web site for the suggested workaround: