The flaw is due to a memory corruption error that occurs when handling or repairing a document containing overly long styles, the French Security Incident Response Team said in an advisory.More details at CNET.
The flaw, which affects Excel 2000, 2002 and 2003 and Office 2000, XP and 2003, "could be exploited by attackers to execute arbitrary commands by convincing a user to open and repair a specially crafted Excel file," the advisory said.
New security flaw found in Excel

A new security hole has been found in Microsoft Excel which allows attackers to take control of a computer.