You have installed Office 2003, 2007, and 2010 on the same computer. Since you cannot run mulitiple versions of Outlook side-by-side, you are using Outlook 2010 only. You have either Excel 2003 or 2007 open when you receive an email message with an Excel attachment. You double-click the attachment, and although Excel comes to the foreground, the file does not open. This issue does not occur if previous versions of Excel are not open, or if Excel 2010 is open. It does not occur with Microsoft Word or PowerPoint attachments.
Microsoft is currently investigating the cause of this issue.
To work around this issue, you can save the file to your hard drive and open it from there. Alternatively, you can open Excel 2010 before you open the earlier version. Then when you double-click the attachment, it will open in Excel 2010.