This indicates an attempt to exploit a remote Memory Corruption vulnerability in Microsoft Excel.
The vulnerability can be exploited via a malformed Excel file containing multiple active worksheets, causing memory corruption. As a result, a remote attacker can crash the application and may also be able to execute arbitrary code on the system.
Microsoft Excel 2003 SP2
Microsoft Excel 2002 SP3
System Compromise: Remote attackers can gain control of vulnerable systems.
Denial of Service: Remote attackers can crash vulnerable systems.
Apply patch, available from the website: