Microsoft Office Excel 2000 SP3, 2002 SP3, 2003 SP3, and 2007 Gold and SP1; Excel Viewer 2003 Gold and SP3; Excel Viewer; Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats Gold and SP1; Office 2004 and 2008 for Mac; and Open XML File Format Converter for Mac allow remote attackers to execute arbitrary code via a crafted Excel spreadsheet that contains a malformed formula, which triggers "pointer corruption" during the loading of formulas from this spreadsheet, aka "File Format Parsing Vulnerability."

http://www.microsoft.com/technet/security/Bulletin/MS08-074.mspx

File Format Parsing Vulnerability - CVE-2008-4264

A remote code execution vulnerability exists in Microsoft Office Excel as a result of pointer corruption when loading Excel formulas. The vulnerability could allow remote code execution if a user opens a specially crafted Excel file that includes a malformed formula. An attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights.