O Office XP está atualizado para o Service Pack mais recente? Relatórios da base de conhecimento da Microsoft :
Issues that occur if Office is not fully updated
The following are issues that may occur if the computer was not updated to be compatible with the 2007 Microsoft Office documents, and you try to open, save, or edit files in the new file format.
When you use the Open dialog box or when you use a drag-and-drop operation to move the file into the version of Excel that you are using, you receive the following error message: The file is not in a recognizable format. When you double-click the file in Windows Explorer, or you double-click the file as an attachment in e-mail, you receive the following error message: Windows cannot open this file.
FWIW Eu também estava enfrentando o mesmo problema, e no meu caso eu pedi ao autor do arquivo para salvá-lo no formato Office 97-2003 e enviá-lo para mim.