Office 2007 Compatibility Pack and .XLSX Issues

Office 2007 Compatibility Pack and .XLSX Issues

I've installed the Microsoft Office 2007 Compatibility Pack and Service Pack 2 on client machines. In one case the user reports he can open .docx files successfully but is unable to open .xlsx files. Has anyone else seen the same issue? If you have can you trace it back to a certain issue? Any input would be appreciated. FYI: the system is XP Pro SP3 running Office XP.